translation: Add Python and Java code for EN version (#1345)

* Add the intial translation of code of all the languages

* test

* revert

* Remove

* Add Python and Java code for EN version

en/codes/java/.gitignore

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+build

en/codes/java/chapter_array_and_linkedlist/array.java

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+/**
+ * File: array.java
+ * Created Time: 2022-11-25
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_array_and_linkedlist;
+
+import java.util.*;
+import java.util.concurrent.ThreadLocalRandom;
+
+public class array {
+    /* Random access to elements */
+    static int randomAccess(int[] nums) {
+        // Randomly select a number in the interval [0, nums.length)
+        int randomIndex = ThreadLocalRandom.current().nextInt(0, nums.length);
+        // Retrieve and return a random element
+        int randomNum = nums[randomIndex];
+        return randomNum;
+    }
+
+    /* Extend array length */
+    static int[] extend(int[] nums, int enlarge) {
+        // Initialize an extended length array
+        int[] res = new int[nums.length + enlarge];
+        // Copy all elements from the original array to the new array
+        for (int i = 0; i < nums.length; i++) {
+            res[i] = nums[i];
+        }
+        // Return the new array after expansion
+        return res;
+    }
+
+    /* Insert element num at `index` */
+    static void insert(int[] nums, int num, int index) {
+        // Move all elements after `index` one position backward
+        for (int i = nums.length - 1; i > index; i--) {
+            nums[i] = nums[i - 1];
+        }
+        // Assign num to the element at index
+        nums[index] = num;
+    }
+
+    /* Remove the element at `index` */
+    static void remove(int[] nums, int index) {
+        // Move all elements after `index` one position forward
+        for (int i = index; i < nums.length - 1; i++) {
+            nums[i] = nums[i + 1];
+        }
+    }
+
+    /* Traverse array */
+    static void traverse(int[] nums) {
+        int count = 0;
+        // Traverse array by index
+        for (int i = 0; i < nums.length; i++) {
+            count += nums[i];
+        }
+        // Traverse array elements
+        for (int num : nums) {
+            count += num;
+        }
+    }
+
+    /* Search for a specified element in the array */
+    static int find(int[] nums, int target) {
+        for (int i = 0; i < nums.length; i++) {
+            if (nums[i] == target)
+                return i;
+        }
+        return -1;
+    }
+
+    /* Driver Code */
+    public static void main(String[] args) {
+        /* Initialize an array */
+        int[] arr = new int[5];
+        System.out.println("Array arr = " + Arrays.toString(arr));
+        int[] nums = { 1, 3, 2, 5, 4 };
+        System.out.println("Array nums = " + Arrays.toString(nums));
+
+        /* Random access */
+        int randomNum = randomAccess(nums);
+        System.out.println("Get a random element from nums = " + randomNum);
+
+        /* Length extension */
+        nums = extend(nums, 3);
+        System.out.println("Extend the array length to 8, resulting in nums = " + Arrays.toString(nums));
+
+        /* Insert element */
+        insert(nums, 6, 3);
+        System.out.println("Insert the number 6 at index 3, resulting in nums = " + Arrays.toString(nums));
+
+        /* Remove element */
+        remove(nums, 2);
+        System.out.println("Remove the element at index 2, resulting in nums = " + Arrays.toString(nums));
+
+        /* Traverse array */
+        traverse(nums);
+
+        /* Search for elements */
+        int index = find(nums, 3);
+        System.out.println("Find element 3 in nums, index = " + index);
+    }
+}

en/codes/java/chapter_array_and_linkedlist/linked_list.java

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+/**
+ * File: linked_list.java
+ * Created Time: 2022-11-25
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_array_and_linkedlist;
+
+import utils.*;
+
+public class linked_list {
+    /* Insert node P after node n0 in the linked list */
+    static void insert(ListNode n0, ListNode P) {
+        ListNode n1 = n0.next;
+        P.next = n1;
+        n0.next = P;
+    }
+
+    /* Remove the first node after node n0 in the linked list */
+    static void remove(ListNode n0) {
+        if (n0.next == null)
+            return;
+        // n0 -> P -> n1
+        ListNode P = n0.next;
+        ListNode n1 = P.next;
+        n0.next = n1;
+    }
+
+    /* Access the node at `index` in the linked list */
+    static ListNode access(ListNode head, int index) {
+        for (int i = 0; i < index; i++) {
+            if (head == null)
+                return null;
+            head = head.next;
+        }
+        return head;
+    }
+
+    /* Search for the first node with value target in the linked list */
+    static int find(ListNode head, int target) {
+        int index = 0;
+        while (head != null) {
+            if (head.val == target)
+                return index;
+            head = head.next;
+            index++;
+        }
+        return -1;
+    }
+
+    /* Driver Code */
+    public static void main(String[] args) {
+        /* Initialize linked list */
+        // Initialize each node
+        ListNode n0 = new ListNode(1);
+        ListNode n1 = new ListNode(3);
+        ListNode n2 = new ListNode(2);
+        ListNode n3 = new ListNode(5);
+        ListNode n4 = new ListNode(4);
+        // Build references between nodes
+        n0.next = n1;
+        n1.next = n2;
+        n2.next = n3;
+        n3.next = n4;
+        System.out.println("The initialized linked list is");
+        PrintUtil.printLinkedList(n0);
+
+        /* Insert node */
+        insert(n0, new ListNode(0));
+        System.out.println("Linked list after inserting the node is");
+        PrintUtil.printLinkedList(n0);
+
+        /* Remove node */
+        remove(n0);
+        System.out.println("Linked list after removing the node is");
+        PrintUtil.printLinkedList(n0);
+
+        /* Access node */
+        ListNode node = access(n0, 3);
+        System.out.println("The value of the node at index 3 in the linked list = " + node.val);
+
+        /* Search node */
+        int index = find(n0, 2);
+        System.out.println("The index of the node with value 2 in the linked list = " + index);
+    }
+}

en/codes/java/chapter_array_and_linkedlist/list.java

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+/**
+ * File: list.java
+ * Created Time: 2022-11-25
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_array_and_linkedlist;
+
+import java.util.*;
+
+public class list {
+    public static void main(String[] args) {
+        /* Initialize list */
+        // The array's element type is Integer[], a wrapper class for int
+        Integer[] numbers = new Integer[] { 1, 3, 2, 5, 4 };
+        List<Integer> nums = new ArrayList<>(Arrays.asList(numbers));
+        System.out.println("List nums = " + nums);
+
+        /* Access element */
+        int num = nums.get(1);
+        System.out.println("Access the element at index 1, obtained num = " + num);
+
+        /* Update element */
+        nums.set(1, 0);
+        System.out.println("Update the element at index 1 to 0, resulting in nums = " + nums);
+
+        /* Clear list */
+        nums.clear();
+        System.out.println("After clearing the list, nums = " + nums);
+
+        /* Add element at the end */
+        nums.add(1);
+        nums.add(3);
+        nums.add(2);
+        nums.add(5);
+        nums.add(4);
+        System.out.println("After adding elements, nums = " + nums);
+
+        /* Insert element in the middle */
+        nums.add(3, 6);
+        System.out.println("Insert the number 6 at index 3, resulting in nums = " + nums);
+
+        /* Remove element */
+        nums.remove(3);
+        System.out.println("Remove the element at index 3, resulting in nums = " + nums);
+
+        /* Traverse the list by index */
+        int count = 0;
+        for (int i = 0; i < nums.size(); i++) {
+            count += nums.get(i);
+        }
+        /* Traverse the list elements */
+        for (int x : nums) {
+            count += x;
+        }
+
+        /* Concatenate two lists */
+        List<Integer> nums1 = new ArrayList<>(Arrays.asList(new Integer[] { 6, 8, 7, 10, 9 }));
+        nums.addAll(nums1);
+        System.out.println("Concatenate list nums1 to nums, resulting in nums = " + nums);
+
+        /* Sort list */
+        Collections.sort(nums);
+        System.out.println("After sorting the list, nums = " + nums);
+    }
+}

en/codes/java/chapter_array_and_linkedlist/my_list.java

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+/**
+ * File: my_list.java
+ * Created Time: 2022-11-25
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_array_and_linkedlist;
+
+import java.util.*;
+
+/* List class */
+class MyList {
+    private int[] arr; // Array (stores list elements)
+    private int capacity = 10; // List capacity
+    private int size = 0; // List length (current number of elements)
+    private int extendRatio = 2; // Multiple for each list expansion
+
+    /* Constructor */
+    public MyList() {
+        arr = new int[capacity];
+    }
+
+    /* Get list length (current number of elements) */
+    public int size() {
+        return size;
+    }
+
+    /* Get list capacity */
+    public int capacity() {
+        return capacity;
+    }
+
+    /* Access element */
+    public int get(int index) {
+        // If the index is out of bounds, throw an exception, as below
+        if (index < 0 || index >= size)
+            throw new IndexOutOfBoundsException("Index out of bounds");
+        return arr[index];
+    }
+
+    /* Update element */
+    public void set(int index, int num) {
+        if (index < 0 || index >= size)
+            throw new IndexOutOfBoundsException("Index out of bounds");
+        arr[index] = num;
+    }
+
+    /* Add element at the end */
+    public void add(int num) {
+        // When the number of elements exceeds capacity, trigger the expansion mechanism
+        if (size == capacity())
+            extendCapacity();
+        arr[size] = num;
+        // Update the number of elements
+        size++;
+    }
+
+    /* Insert element in the middle */
+    public void insert(int index, int num) {
+        if (index < 0 || index >= size)
+            throw new IndexOutOfBoundsException("Index out of bounds");
+        // When the number of elements exceeds capacity, trigger the expansion mechanism
+        if (size == capacity())
+            extendCapacity();
+        // Move all elements after `index` one position backward
+        for (int j = size - 1; j >= index; j--) {
+            arr[j + 1] = arr[j];
+        }
+        arr[index] = num;
+        // Update the number of elements
+        size++;
+    }
+
+    /* Remove element */
+    public int remove(int index) {
+        if (index < 0 || index >= size)
+            throw new IndexOutOfBoundsException("Index out of bounds");
+        int num = arr[index];
+        // Move all elements after `index` one position forward
+        for (int j = index; j < size - 1; j++) {
+            arr[j] = arr[j + 1];
+        }
+        // Update the number of elements
+        size--;
+        // Return the removed element
+        return num;
+    }
+
+    /* Extend list */
+    public void extendCapacity() {
+        // Create a new array with a length multiple of the original array by extendRatio, and copy the original array to the new array
+        arr = Arrays.copyOf(arr, capacity() * extendRatio);
+        // Update list capacity
+        capacity = arr.length;
+    }
+
+    /* Convert the list to an array */
+    public int[] toArray() {
+        int size = size();
+        // Only convert elements within valid length range
+        int[] arr = new int[size];
+        for (int i = 0; i < size; i++) {
+            arr[i] = get(i);
+        }
+        return arr;
+    }
+}
+
+public class my_list {
+    /* Driver Code */
+    public static void main(String[] args) {
+        /* Initialize list */
+        MyList nums = new MyList();
+        /* Add element at the end */
+        nums.add(1);
+        nums.add(3);
+        nums.add(2);
+        nums.add(5);
+        nums.add(4);
+        System.out.println("List nums = " + Arrays.toString(nums.toArray()) +
+                ", capacity = " + nums.capacity() + ", length = " + nums.size());
+
+        /* Insert element in the middle */
+        nums.insert(3, 6);
+        System.out.println("Insert the number 6 at index 3, resulting in nums = " + Arrays.toString(nums.toArray()));
+
+        /* Remove element */
+        nums.remove(3);
+        System.out.println("Remove the element at index 3, resulting in nums = " + Arrays.toString(nums.toArray()));
+
+        /* Access element */
+        int num = nums.get(1);
+        System.out.println("Access the element at index 1, obtained num = " + num);
+
+        /* Update element */
+        nums.set(1, 0);
+        System.out.println("Update the element at index 1 to 0, resulting in nums = " + Arrays.toString(nums.toArray()));
+
+        /* Test expansion mechanism */
+        for (int i = 0; i < 10; i++) {
+            // At i = 5, the list length will exceed the list capacity, triggering the expansion mechanism at this time
+            nums.add(i);
+        }
+        System.out.println("After extending, list nums = " + Arrays.toString(nums.toArray()) +
+                ", capacity = " + nums.capacity() + ", length = " + nums.size());
+    }
+}

en/codes/java/chapter_backtracking/n_queens.java

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+/**
+ * File: n_queens.java
+ * Created Time: 2023-05-04
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_backtracking;
+
+import java.util.*;
+
+public class n_queens {
+    /* Backtracking algorithm: n queens */
+    public static void backtrack(int row, int n, List<List<String>> state, List<List<List<String>>> res,
+            boolean[] cols, boolean[] diags1, boolean[] diags2) {
+        // When all rows are placed, record the solution
+        if (row == n) {
+            List<List<String>> copyState = new ArrayList<>();
+            for (List<String> sRow : state) {
+                copyState.add(new ArrayList<>(sRow));
+            }
+            res.add(copyState);
+            return;
+        }
+        // Traverse all columns
+        for (int col = 0; col < n; col++) {
+            // Calculate the main and minor diagonals corresponding to the cell
+            int diag1 = row - col + n - 1;
+            int diag2 = row + col;
+            // Pruning: do not allow queens on the column, main diagonal, or minor diagonal of the cell
+            if (!cols[col] && !diags1[diag1] && !diags2[diag2]) {
+                // Attempt: place the queen in the cell
+                state.get(row).set(col, "Q");
+                cols[col] = diags1[diag1] = diags2[diag2] = true;
+                // Place the next row
+                backtrack(row + 1, n, state, res, cols, diags1, diags2);
+                // Retract: restore the cell to an empty spot
+                state.get(row).set(col, "#");
+                cols[col] = diags1[diag1] = diags2[diag2] = false;
+            }
+        }
+    }
+
+    /* Solve n queens */
+    public static List<List<List<String>>> nQueens(int n) {
+        // Initialize an n*n size chessboard, where 'Q' represents the queen and '#' represents an empty spot
+        List<List<String>> state = new ArrayList<>();
+        for (int i = 0; i < n; i++) {
+            List<String> row = new ArrayList<>();
+            for (int j = 0; j < n; j++) {
+                row.add("#");
+            }
+            state.add(row);
+        }
+        boolean[] cols = new boolean[n]; // Record columns with queens
+        boolean[] diags1 = new boolean[2 * n - 1]; // Record main diagonals with queens
+        boolean[] diags2 = new boolean[2 * n - 1]; // Record minor diagonals with queens
+        List<List<List<String>>> res = new ArrayList<>();
+
+        backtrack(0, n, state, res, cols, diags1, diags2);
+
+        return res;
+    }
+
+    public static void main(String[] args) {
+        int n = 4;
+        List<List<List<String>>> res = nQueens(n);
+
+        System.out.println("Input the dimensions of the chessboard as " + n);
+        System.out.println("Total number of queen placement solutions = " + res.size());
+        for (List<List<String>> state : res) {
+            System.out.println("--------------------");
+            for (List<String> row : state) {
+                System.out.println(row);
+            }
+        }
+    }
+}

en/codes/java/chapter_backtracking/permutations_i.java

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+/**
+ * File: permutations_i.java
+ * Created Time: 2023-04-24
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_backtracking;
+
+import java.util.*;
+
+public class permutations_i {
+    /* Backtracking algorithm: Permutation I */
+    public static void backtrack(List<Integer> state, int[] choices, boolean[] selected, List<List<Integer>> res) {
+        // When the state length equals the number of elements, record the solution
+        if (state.size() == choices.length) {
+            res.add(new ArrayList<Integer>(state));
+            return;
+        }
+        // Traverse all choices
+        for (int i = 0; i < choices.length; i++) {
+            int choice = choices[i];
+            // Pruning: do not allow repeated selection of elements
+            if (!selected[i]) {
+                // Attempt: make a choice, update the state
+                selected[i] = true;
+                state.add(choice);
+                // Proceed to the next round of selection
+                backtrack(state, choices, selected, res);
+                // Retract: undo the choice, restore to the previous state
+                selected[i] = false;
+                state.remove(state.size() - 1);
+            }
+        }
+    }
+
+    /* Permutation I */
+    static List<List<Integer>> permutationsI(int[] nums) {
+        List<List<Integer>> res = new ArrayList<List<Integer>>();
+        backtrack(new ArrayList<Integer>(), nums, new boolean[nums.length], res);
+        return res;
+    }
+
+    public static void main(String[] args) {
+        int[] nums = { 1, 2, 3 };
+
+        List<List<Integer>> res = permutationsI(nums);
+
+        System.out.println("Input array nums = " + Arrays.toString(nums));
+        System.out.println("All permutations res = " + res);
+    }
+}

en/codes/java/chapter_backtracking/permutations_ii.java

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+/**
+ * File: permutations_ii.java
+ * Created Time: 2023-04-24
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_backtracking;
+
+import java.util.*;
+
+public class permutations_ii {
+    /* Backtracking algorithm: Permutation II */
+    static void backtrack(List<Integer> state, int[] choices, boolean[] selected, List<List<Integer>> res) {
+        // When the state length equals the number of elements, record the solution
+        if (state.size() == choices.length) {
+            res.add(new ArrayList<Integer>(state));
+            return;
+        }
+        // Traverse all choices
+        Set<Integer> duplicated = new HashSet<Integer>();
+        for (int i = 0; i < choices.length; i++) {
+            int choice = choices[i];
+            // Pruning: do not allow repeated selection of elements and do not allow repeated selection of equal elements
+            if (!selected[i] && !duplicated.contains(choice)) {
+                // Attempt: make a choice, update the state
+                duplicated.add(choice); // Record selected element values
+                selected[i] = true;
+                state.add(choice);
+                // Proceed to the next round of selection
+                backtrack(state, choices, selected, res);
+                // Retract: undo the choice, restore to the previous state
+                selected[i] = false;
+                state.remove(state.size() - 1);
+            }
+        }
+    }
+
+    /* Permutation II */
+    static List<List<Integer>> permutationsII(int[] nums) {
+        List<List<Integer>> res = new ArrayList<List<Integer>>();
+        backtrack(new ArrayList<Integer>(), nums, new boolean[nums.length], res);
+        return res;
+    }
+
+    public static void main(String[] args) {
+        int[] nums = { 1, 2, 2 };
+
+        List<List<Integer>> res = permutationsII(nums);
+
+        System.out.println("Input array nums = " + Arrays.toString(nums));
+        System.out.println("All permutations res = " + res);
+    }
+}

en/codes/java/chapter_backtracking/preorder_traversal_i_compact.java

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+/**
+ * File: preorder_traversal_i_compact.java
+ * Created Time: 2023-04-16
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_backtracking;
+
+import utils.*;
+import java.util.*;
+
+public class preorder_traversal_i_compact {
+    static List<TreeNode> res;
+
+    /* Pre-order traversal: Example one */
+    static void preOrder(TreeNode root) {
+        if (root == null) {
+            return;
+        }
+        if (root.val == 7) {
+            // Record solution
+            res.add(root);
+        }
+        preOrder(root.left);
+        preOrder(root.right);
+    }
+
+    public static void main(String[] args) {
+        TreeNode root = TreeNode.listToTree(Arrays.asList(1, 7, 3, 4, 5, 6, 7));
+        System.out.println("\nInitialize binary tree");
+        PrintUtil.printTree(root);
+
+        // Pre-order traversal
+        res = new ArrayList<>();
+        preOrder(root);
+
+        System.out.println("\nOutput all nodes with value 7");
+        List<Integer> vals = new ArrayList<>();
+        for (TreeNode node : res) {
+            vals.add(node.val);
+        }
+        System.out.println(vals);
+    }
+}

en/codes/java/chapter_backtracking/preorder_traversal_ii_compact.java

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+/**
+ * File: preorder_traversal_ii_compact.java
+ * Created Time: 2023-04-16
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_backtracking;
+
+import utils.*;
+import java.util.*;
+
+public class preorder_traversal_ii_compact {
+    static List<TreeNode> path;
+    static List<List<TreeNode>> res;
+
+    /* Pre-order traversal: Example two */
+    static void preOrder(TreeNode root) {
+        if (root == null) {
+            return;
+        }
+        // Attempt
+        path.add(root);
+        if (root.val == 7) {
+            // Record solution
+            res.add(new ArrayList<>(path));
+        }
+        preOrder(root.left);
+        preOrder(root.right);
+        // Retract
+        path.remove(path.size() - 1);
+    }
+
+    public static void main(String[] args) {
+        TreeNode root = TreeNode.listToTree(Arrays.asList(1, 7, 3, 4, 5, 6, 7));
+        System.out.println("\nInitialize binary tree");
+        PrintUtil.printTree(root);
+
+        // Pre-order traversal
+        path = new ArrayList<>();
+        res = new ArrayList<>();
+        preOrder(root);
+
+        System.out.println("\nOutput all root-to-node 7 paths");
+        for (List<TreeNode> path : res) {
+            List<Integer> vals = new ArrayList<>();
+            for (TreeNode node : path) {
+                vals.add(node.val);
+            }
+            System.out.println(vals);
+        }
+    }
+}

en/codes/java/chapter_backtracking/preorder_traversal_iii_compact.java

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+/**
+ * File: preorder_traversal_iii_compact.java
+ * Created Time: 2023-04-16
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_backtracking;
+
+import utils.*;
+import java.util.*;
+
+public class preorder_traversal_iii_compact {
+    static List<TreeNode> path;
+    static List<List<TreeNode>> res;
+
+    /* Pre-order traversal: Example three */
+    static void preOrder(TreeNode root) {
+        // Pruning
+        if (root == null || root.val == 3) {
+            return;
+        }
+        // Attempt
+        path.add(root);
+        if (root.val == 7) {
+            // Record solution
+            res.add(new ArrayList<>(path));
+        }
+        preOrder(root.left);
+        preOrder(root.right);
+        // Retract
+        path.remove(path.size() - 1);
+    }
+
+    public static void main(String[] args) {
+        TreeNode root = TreeNode.listToTree(Arrays.asList(1, 7, 3, 4, 5, 6, 7));
+        System.out.println("\nInitialize binary tree");
+        PrintUtil.printTree(root);
+
+        // Pre-order traversal
+        path = new ArrayList<>();
+        res = new ArrayList<>();
+        preOrder(root);
+
+        System.out.println("\nOutput all root-to-node 7 paths, not including nodes with value 3");
+        for (List<TreeNode> path : res) {
+            List<Integer> vals = new ArrayList<>();
+            for (TreeNode node : path) {
+                vals.add(node.val);
+            }
+            System.out.println(vals);
+        }
+    }
+}

en/codes/java/chapter_backtracking/preorder_traversal_iii_template.java

@@ -0,0 +1,77 @@
+/**
+ * File: preorder_traversal_iii_template.java
+ * Created Time: 2023-04-16
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_backtracking;
+
+import utils.*;
+import java.util.*;
+
+public class preorder_traversal_iii_template {
+    /* Determine if the current state is a solution */
+    static boolean isSolution(List<TreeNode> state) {
+        return !state.isEmpty() && state.get(state.size() - 1).val == 7;
+    }
+
+    /* Record solution */
+    static void recordSolution(List<TreeNode> state, List<List<TreeNode>> res) {
+        res.add(new ArrayList<>(state));
+    }
+
+    /* Determine if the choice is legal under the current state */
+    static boolean isValid(List<TreeNode> state, TreeNode choice) {
+        return choice != null && choice.val != 3;
+    }
+
+    /* Update state */
+    static void makeChoice(List<TreeNode> state, TreeNode choice) {
+        state.add(choice);
+    }
+
+    /* Restore state */
+    static void undoChoice(List<TreeNode> state, TreeNode choice) {
+        state.remove(state.size() - 1);
+    }
+
+    /* Backtracking algorithm: Example three */
+    static void backtrack(List<TreeNode> state, List<TreeNode> choices, List<List<TreeNode>> res) {
+        // Check if it's a solution
+        if (isSolution(state)) {
+            // Record solution
+            recordSolution(state, res);
+        }
+        // Traverse all choices
+        for (TreeNode choice : choices) {
+            // Pruning: check if the choice is legal
+            if (isValid(state, choice)) {
+                // Attempt: make a choice, update the state
+                makeChoice(state, choice);
+                // Proceed to the next round of selection
+                backtrack(state, Arrays.asList(choice.left, choice.right), res);
+                // Retract: undo the choice, restore to the previous state
+                undoChoice(state, choice);
+            }
+        }
+    }
+
+    public static void main(String[] args) {
+        TreeNode root = TreeNode.listToTree(Arrays.asList(1, 7, 3, 4, 5, 6, 7));
+        System.out.println("\nInitialize binary tree");
+        PrintUtil.printTree(root);
+
+        // Backtracking algorithm
+        List<List<TreeNode>> res = new ArrayList<>();
+        backtrack(new ArrayList<>(), Arrays.asList(root), res);
+
+        System.out.println("\nOutput all root-to-node 7 paths, requiring paths not to include nodes with value 3");
+        for (List<TreeNode> path : res) {
+            List<Integer> vals = new ArrayList<>();
+            for (TreeNode node : path) {
+                vals.add(node.val);
+            }
+            System.out.println(vals);
+        }
+    }
+}

en/codes/java/chapter_backtracking/subset_sum_i.java

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+/**
+ * File: subset_sum_i.java
+ * Created Time: 2023-06-21
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_backtracking;
+
+import java.util.*;
+
+public class subset_sum_i {
+    /* Backtracking algorithm: Subset Sum I */
+    static void backtrack(List<Integer> state, int target, int[] choices, int start, List<List<Integer>> res) {
+        // When the subset sum equals target, record the solution
+        if (target == 0) {
+            res.add(new ArrayList<>(state));
+            return;
+        }
+        // Traverse all choices
+        // Pruning two: start traversing from start to avoid generating duplicate subsets
+        for (int i = start; i < choices.length; i++) {
+            // Pruning one: if the subset sum exceeds target, end the loop immediately
+            // This is because the array is sorted, and later elements are larger, so the subset sum will definitely exceed target
+            if (target - choices[i] < 0) {
+                break;
+            }
+            // Attempt: make a choice, update target, start
+            state.add(choices[i]);
+            // Proceed to the next round of selection
+            backtrack(state, target - choices[i], choices, i, res);
+            // Retract: undo the choice, restore to the previous state
+            state.remove(state.size() - 1);
+        }
+    }
+
+    /* Solve Subset Sum I */
+    static List<List<Integer>> subsetSumI(int[] nums, int target) {
+        List<Integer> state = new ArrayList<>(); // State (subset)
+        Arrays.sort(nums); // Sort nums
+        int start = 0; // Start point for traversal
+        List<List<Integer>> res = new ArrayList<>(); // Result list (subset list)
+        backtrack(state, target, nums, start, res);
+        return res;
+    }
+
+    public static void main(String[] args) {
+        int[] nums = { 3, 4, 5 };
+        int target = 9;
+
+        List<List<Integer>> res = subsetSumI(nums, target);
+
+        System.out.println("Input array nums = " + Arrays.toString(nums) + ", target = " + target);
+        System.out.println("All subsets summing to " + target + " res = " + res);
+    }
+}

en/codes/java/chapter_backtracking/subset_sum_i_naive.java

@@ -0,0 +1,53 @@
+/**
+ * File: subset_sum_i_naive.java
+ * Created Time: 2023-06-21
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_backtracking;
+
+import java.util.*;
+
+public class subset_sum_i_naive {
+    /* Backtracking algorithm: Subset Sum I */
+    static void backtrack(List<Integer> state, int target, int total, int[] choices, List<List<Integer>> res) {
+        // When the subset sum equals target, record the solution
+        if (total == target) {
+            res.add(new ArrayList<>(state));
+            return;
+        }
+        // Traverse all choices
+        for (int i = 0; i < choices.length; i++) {
+            // Pruning: if the subset sum exceeds target, skip that choice
+            if (total + choices[i] > target) {
+                continue;
+            }
+            // Attempt: make a choice, update elements and total
+            state.add(choices[i]);
+            // Proceed to the next round of selection
+            backtrack(state, target, total + choices[i], choices, res);
+            // Retract: undo the choice, restore to the previous state
+            state.remove(state.size() - 1);
+        }
+    }
+
+    /* Solve Subset Sum I (including duplicate subsets) */
+    static List<List<Integer>> subsetSumINaive(int[] nums, int target) {
+        List<Integer> state = new ArrayList<>(); // State (subset)
+        int total = 0; // Subset sum
+        List<List<Integer>> res = new ArrayList<>(); // Result list (subset list)
+        backtrack(state, target, total, nums, res);
+        return res;
+    }
+
+    public static void main(String[] args) {
+        int[] nums = { 3, 4, 5 };
+        int target = 9;
+
+        List<List<Integer>> res = subsetSumINaive(nums, target);
+
+        System.out.println("Input array nums = " + Arrays.toString(nums) + ", target = " + target);
+        System.out.println("All subsets summing to " + target + " res = " + res);
+        System.out.println("Please note that the result of this method includes duplicate sets");
+    }
+}

en/codes/java/chapter_backtracking/subset_sum_ii.java

@@ -0,0 +1,60 @@
+/**
+ * File: subset_sum_ii.java
+ * Created Time: 2023-06-21
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_backtracking;
+
+import java.util.*;
+
+public class subset_sum_ii {
+    /* Backtracking algorithm: Subset Sum II */
+    static void backtrack(List<Integer> state, int target, int[] choices, int start, List<List<Integer>> res) {
+        // When the subset sum equals target, record the solution
+        if (target == 0) {
+            res.add(new ArrayList<>(state));
+            return;
+        }
+        // Traverse all choices
+        // Pruning two: start traversing from start to avoid generating duplicate subsets
+        // Pruning three: start traversing from start to avoid repeatedly selecting the same element
+        for (int i = start; i < choices.length; i++) {
+            // Pruning one: if the subset sum exceeds target, end the loop immediately
+            // This is because the array is sorted, and later elements are larger, so the subset sum will definitely exceed target
+            if (target - choices[i] < 0) {
+                break;
+            }
+            // Pruning four: if the element equals the left element, it indicates that the search branch is repeated, skip it
+            if (i > start && choices[i] == choices[i - 1]) {
+                continue;
+            }
+            // Attempt: make a choice, update target, start
+            state.add(choices[i]);
+            // Proceed to the next round of selection
+            backtrack(state, target - choices[i], choices, i + 1, res);
+            // Retract: undo the choice, restore to the previous state
+            state.remove(state.size() - 1);
+        }
+    }
+
+    /* Solve Subset Sum II */
+    static List<List<Integer>> subsetSumII(int[] nums, int target) {
+        List<Integer> state = new ArrayList<>(); // State (subset)
+        Arrays.sort(nums); // Sort nums
+        int start = 0; // Start point for traversal
+        List<List<Integer>> res = new ArrayList<>(); // Result list (subset list)
+        backtrack(state, target, nums, start, res);
+        return res;
+    }
+
+    public static void main(String[] args) {
+        int[] nums = { 4, 4, 5 };
+        int target = 9;
+
+        List<List<Integer>> res = subsetSumII(nums, target);
+
+        System.out.println("Input array nums = " + Arrays.toString(nums) + ", target = " + target);
+        System.out.println("All subsets summing to " + target + " res = " + res);
+    }
+}

en/codes/java/chapter_computational_complexity/iteration.java

@@ -0,0 +1,76 @@
+/**
+ * File: iteration.java
+ * Created Time: 2023-08-24
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_computational_complexity;
+
+public class iteration {
+    /* for loop */
+    static int forLoop(int n) {
+        int res = 0;
+        // Loop sum 1, 2, ..., n-1, n
+        for (int i = 1; i <= n; i++) {
+            res += i;
+        }
+        return res;
+    }
+
+    /* while loop */
+    static int whileLoop(int n) {
+        int res = 0;
+        int i = 1; // Initialize condition variable
+        // Loop sum 1, 2, ..., n-1, n
+        while (i <= n) {
+            res += i;
+            i++; // Update condition variable
+        }
+        return res;
+    }
+
+    /* while loop (two updates) */
+    static int whileLoopII(int n) {
+        int res = 0;
+        int i = 1; // Initialize condition variable
+        // Loop sum 1, 4, 10, ...
+        while (i <= n) {
+            res += i;
+            // Update condition variable
+            i++;
+            i *= 2;
+        }
+        return res;
+    }
+
+    /* Double for loop */
+    static String nestedForLoop(int n) {
+        StringBuilder res = new StringBuilder();
+        // Loop i = 1, 2, ..., n-1, n
+        for (int i = 1; i <= n; i++) {
+            // Loop j = 1, 2, ..., n-1, n
+            for (int j = 1; j <= n; j++) {
+                res.append("(" + i + ", " + j + "), ");
+            }
+        }
+        return res.toString();
+    }
+
+    /* Driver Code */
+    public static void main(String[] args) {
+        int n = 5;
+        int res;
+
+        res = forLoop(n);
+        System.out.println("\nSum result of the for loop res = " + res);
+
+        res = whileLoop(n);
+        System.out.println("\nSum result of the while loop res = " + res);
+
+        res = whileLoopII(n);
+        System.out.println("\nSum result of the while loop (with two updates) res = " + res);
+
+        String resStr = nestedForLoop(n);
+        System.out.println("\nResult of the double for loop traversal = " + resStr);
+    }
+}

en/codes/java/chapter_computational_complexity/recursion.java

@@ -0,0 +1,79 @@
+/**
+ * File: recursion.java
+ * Created Time: 2023-08-24
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_computational_complexity;
+
+import java.util.Stack;
+
+public class recursion {
+    /* Recursion */
+    static int recur(int n) {
+        // Termination condition
+        if (n == 1)
+            return 1;
+        // Recursive: recursive call
+        int res = recur(n - 1);
+        // Return: return result
+        return n + res;
+    }
+
+    /* Simulate recursion with iteration */
+    static int forLoopRecur(int n) {
+        // Use an explicit stack to simulate the system call stack
+        Stack<Integer> stack = new Stack<>();
+        int res = 0;
+        // Recursive: recursive call
+        for (int i = n; i > 0; i--) {
+            // Simulate "recursive" by "pushing onto the stack"
+            stack.push(i);
+        }
+        // Return: return result
+        while (!stack.isEmpty()) {
+            // Simulate "return" by "popping from the stack"
+            res += stack.pop();
+        }
+        // res = 1+2+3+...+n
+        return res;
+    }
+
+    /* Tail recursion */
+    static int tailRecur(int n, int res) {
+        // Termination condition
+        if (n == 0)
+            return res;
+        // Tail recursive call
+        return tailRecur(n - 1, res + n);
+    }
+
+    /* Fibonacci sequence: Recursion */
+    static int fib(int n) {
+        // Termination condition f(1) = 0, f(2) = 1
+        if (n == 1 || n == 2)
+            return n - 1;
+        // Recursive call f(n) = f(n-1) + f(n-2)
+        int res = fib(n - 1) + fib(n - 2);
+        // Return result f(n)
+        return res;
+    }
+
+    /* Driver Code */
+    public static void main(String[] args) {
+        int n = 5;
+        int res;
+
+        res = recur(n);
+        System.out.println("\nSum result of the recursive function res = " + res);
+
+        res = forLoopRecur(n);
+        System.out.println("\nSum result using iteration to simulate recursion res = " + res);
+
+        res = tailRecur(n, 0);
+        System.out.println("\nSum result of the tail-recursive function res = " + res);
+
+        res = fib(n);
+        System.out.println("\nThe " + n + "th number in the Fibonacci sequence is " + res);
+    }
+}

en/codes/java/chapter_computational_complexity/space_complexity.java

@@ -0,0 +1,110 @@
+/**
+ * File: space_complexity.java
+ * Created Time: 2022-11-25
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_computational_complexity;
+
+import utils.*;
+import java.util.*;
+
+public class space_complexity {
+    /* Function */
+    static int function() {
+        // Perform some operations
+        return 0;
+    }
+
+    /* Constant complexity */
+    static void constant(int n) {
+        // Constants, variables, objects occupy O(1) space
+        final int a = 0;
+        int b = 0;
+        int[] nums = new int[10000];
+        ListNode node = new ListNode(0);
+        // Variables in a loop occupy O(1) space
+        for (int i = 0; i < n; i++) {
+            int c = 0;
+        }
+        // Functions in a loop occupy O(1) space
+        for (int i = 0; i < n; i++) {
+            function();
+        }
+    }
+
+    /* Linear complexity */
+    static void linear(int n) {
+        // Array of length n occupies O(n) space
+        int[] nums = new int[n];
+        // A list of length n occupies O(n) space
+        List<ListNode> nodes = new ArrayList<>();
+        for (int i = 0; i < n; i++) {
+            nodes.add(new ListNode(i));
+        }
+        // A hash table of length n occupies O(n) space
+        Map<Integer, String> map = new HashMap<>();
+        for (int i = 0; i < n; i++) {
+            map.put(i, String.valueOf(i));
+        }
+    }
+
+    /* Linear complexity (recursive implementation) */
+    static void linearRecur(int n) {
+        System.out.println("Recursion n = " + n);
+        if (n == 1)
+            return;
+        linearRecur(n - 1);
+    }
+
+    /* Quadratic complexity */
+    static void quadratic(int n) {
+        // Matrix occupies O(n^2) space
+        int[][] numMatrix = new int[n][n];
+        // A two-dimensional list occupies O(n^2) space
+        List<List<Integer>> numList = new ArrayList<>();
+        for (int i = 0; i < n; i++) {
+            List<Integer> tmp = new ArrayList<>();
+            for (int j = 0; j < n; j++) {
+                tmp.add(0);
+            }
+            numList.add(tmp);
+        }
+    }
+
+    /* Quadratic complexity (recursive implementation) */
+    static int quadraticRecur(int n) {
+        if (n <= 0)
+            return 0;
+        // Array nums length = n, n-1, ..., 2, 1
+        int[] nums = new int[n];
+        System.out.println("Recursion n = " + n + " in the length of nums = " + nums.length);
+        return quadraticRecur(n - 1);
+    }
+
+    /* Exponential complexity (building a full binary tree) */
+    static TreeNode buildTree(int n) {
+        if (n == 0)
+            return null;
+        TreeNode root = new TreeNode(0);
+        root.left = buildTree(n - 1);
+        root.right = buildTree(n - 1);
+        return root;
+    }
+
+    /* Driver Code */
+    public static void main(String[] args) {
+        int n = 5;
+        // Constant complexity
+        constant(n);
+        // Linear complexity
+        linear(n);
+        linearRecur(n);
+        // Quadratic complexity
+        quadratic(n);
+        quadraticRecur(n);
+        // Exponential complexity
+        TreeNode root = buildTree(n);
+        PrintUtil.printTree(root);
+    }
+}

en/codes/java/chapter_computational_complexity/time_complexity.java

@@ -0,0 +1,167 @@
+/**
+ * File: time_complexity.java
+ * Created Time: 2022-11-25
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_computational_complexity;
+
+public class time_complexity {
+    /* Constant complexity */
+    static int constant(int n) {
+        int count = 0;
+        int size = 100000;
+        for (int i = 0; i < size; i++)
+            count++;
+        return count;
+    }
+
+    /* Linear complexity */
+    static int linear(int n) {
+        int count = 0;
+        for (int i = 0; i < n; i++)
+            count++;
+        return count;
+    }
+
+    /* Linear complexity (traversing an array) */
+    static int arrayTraversal(int[] nums) {
+        int count = 0;
+        // Loop count is proportional to the length of the array
+        for (int num : nums) {
+            count++;
+        }
+        return count;
+    }
+
+    /* Quadratic complexity */
+    static int quadratic(int n) {
+        int count = 0;
+        // Loop count is squared in relation to the data size n
+        for (int i = 0; i < n; i++) {
+            for (int j = 0; j < n; j++) {
+                count++;
+            }
+        }
+        return count;
+    }
+
+    /* Quadratic complexity (bubble sort) */
+    static int bubbleSort(int[] nums) {
+        int count = 0; // Counter
+        // Outer loop: unsorted range is [0, i]
+        for (int i = nums.length - 1; i > 0; i--) {
+            // Inner loop: swap the largest element in the unsorted range [0, i] to the right end of the range
+            for (int j = 0; j < i; j++) {
+                if (nums[j] > nums[j + 1]) {
+                    // Swap nums[j] and nums[j + 1]
+                    int tmp = nums[j];
+                    nums[j] = nums[j + 1];
+                    nums[j + 1] = tmp;
+                    count += 3; // Element swap includes 3 individual operations
+                }
+            }
+        }
+        return count;
+    }
+
+    /* Exponential complexity (loop implementation) */
+    static int exponential(int n) {
+        int count = 0, base = 1;
+        // Cells split into two every round, forming the sequence 1, 2, 4, 8, ..., 2^(n-1)
+        for (int i = 0; i < n; i++) {
+            for (int j = 0; j < base; j++) {
+                count++;
+            }
+            base *= 2;
+        }
+        // count = 1 + 2 + 4 + 8 + .. + 2^(n-1) = 2^n - 1
+        return count;
+    }
+
+    /* Exponential complexity (recursive implementation) */
+    static int expRecur(int n) {
+        if (n == 1)
+            return 1;
+        return expRecur(n - 1) + expRecur(n - 1) + 1;
+    }
+
+    /* Logarithmic complexity (loop implementation) */
+    static int logarithmic(int n) {
+        int count = 0;
+        while (n > 1) {
+            n = n / 2;
+            count++;
+        }
+        return count;
+    }
+
+    /* Logarithmic complexity (recursive implementation) */
+    static int logRecur(int n) {
+        if (n <= 1)
+            return 0;
+        return logRecur(n / 2) + 1;
+    }
+
+    /* Linear logarithmic complexity */
+    static int linearLogRecur(int n) {
+        if (n <= 1)
+            return 1;
+        int count = linearLogRecur(n / 2) + linearLogRecur(n / 2);
+        for (int i = 0; i < n; i++) {
+            count++;
+        }
+        return count;
+    }
+
+    /* Factorial complexity (recursive implementation) */
+    static int factorialRecur(int n) {
+        if (n == 0)
+            return 1;
+        int count = 0;
+        // From 1 split into n
+        for (int i = 0; i < n; i++) {
+            count += factorialRecur(n - 1);
+        }
+        return count;
+    }
+
+    /* Driver Code */
+    public static void main(String[] args) {
+        // Can modify n to experience the trend of operation count changes under various complexities
+        int n = 8;
+        System.out.println("Input data size n = " + n);
+
+        int count = constant(n);
+        System.out.println("Number of constant complexity operations = " + count);
+
+        count = linear(n);
+        System.out.println("Number of linear complexity operations = " + count);
+        count = arrayTraversal(new int[n]);
+        System.out.println("Number of linear complexity operations (traversing the array) = " + count);
+
+        count = quadratic(n);
+        System.out.println("Number of quadratic order operations = " + count);
+        int[] nums = new int[n];
+        for (int i = 0; i < n; i++)
+            nums[i] = n - i; // [n,n-1,...,2,1]
+        count = bubbleSort(nums);
+        System.out.println("Number of quadratic order operations (bubble sort) = " + count);
+
+        count = exponential(n);
+        System.out.println("Number of exponential complexity operations (implemented by loop) = " + count);
+        count = expRecur(n);
+        System.out.println("Number of exponential complexity operations (implemented by recursion) = " + count);
+
+        count = logarithmic(n);
+        System.out.println("Number of logarithmic complexity operations (implemented by loop) = " + count);
+        count = logRecur(n);
+        System.out.println("Number of logarithmic complexity operations (implemented by recursion) = " + count);
+
+        count = linearLogRecur(n);
+        System.out.println("Number of linear logarithmic complexity operations (implemented by recursion) = " + count);
+
+        count = factorialRecur(n);
+        System.out.println("Number of factorial complexity operations (implemented by recursion) = " + count);
+    }
+}

en/codes/java/chapter_computational_complexity/worst_best_time_complexity.java

@@ -0,0 +1,50 @@
+/**
+ * File: worst_best_time_complexity.java
+ * Created Time: 2022-11-25
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_computational_complexity;
+
+import java.util.*;
+
+public class worst_best_time_complexity {
+    /* Generate an array with elements {1, 2, ..., n} in a randomly shuffled order */
+    static int[] randomNumbers(int n) {
+        Integer[] nums = new Integer[n];
+        // Generate array nums = { 1, 2, 3, ..., n }
+        for (int i = 0; i < n; i++) {
+            nums[i] = i + 1;
+        }
+        // Randomly shuffle array elements
+        Collections.shuffle(Arrays.asList(nums));
+        // Integer[] -> int[]
+        int[] res = new int[n];
+        for (int i = 0; i < n; i++) {
+            res[i] = nums[i];
+        }
+        return res;
+    }
+
+    /* Find the index of number 1 in array nums */
+    static int findOne(int[] nums) {
+        for (int i = 0; i < nums.length; i++) {
+            // When element 1 is at the start of the array, achieve best time complexity O(1)
+            // When element 1 is at the end of the array, achieve worst time complexity O(n)
+            if (nums[i] == 1)
+                return i;
+        }
+        return -1;
+    }
+
+    /* Driver Code */
+    public static void main(String[] args) {
+        for (int i = 0; i < 10; i++) {
+            int n = 100;
+            int[] nums = randomNumbers(n);
+            int index = findOne(nums);
+            System.out.println("\nThe array [ 1, 2, ..., n ] after being shuffled = " + Arrays.toString(nums));
+            System.out.println("The index of number 1 is " + index);
+        }
+    }
+}

en/codes/java/chapter_divide_and_conquer/binary_search_recur.java

@@ -0,0 +1,45 @@
+/**
+ * File: binary_search_recur.java
+ * Created Time: 2023-07-17
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_divide_and_conquer;
+
+public class binary_search_recur {
+    /* Binary search: problem f(i, j) */
+    static int dfs(int[] nums, int target, int i, int j) {
+        // If the interval is empty, indicating no target element, return -1
+        if (i > j) {
+            return -1;
+        }
+        // Calculate midpoint index m
+        int m = (i + j) / 2;
+        if (nums[m] < target) {
+            // Recursive subproblem f(m+1, j)
+            return dfs(nums, target, m + 1, j);
+        } else if (nums[m] > target) {
+            // Recursive subproblem f(i, m-1)
+            return dfs(nums, target, i, m - 1);
+        } else {
+            // Found the target element, thus return its index
+            return m;
+        }
+    }
+
+    /* Binary search */
+    static int binarySearch(int[] nums, int target) {
+        int n = nums.length;
+        // Solve problem f(0, n-1)
+        return dfs(nums, target, 0, n - 1);
+    }
+
+    public static void main(String[] args) {
+        int target = 6;
+        int[] nums = { 1, 3, 6, 8, 12, 15, 23, 26, 31, 35 };
+
+        // Binary search (double closed interval)
+        int index = binarySearch(nums, target);
+        System.out.println("Index of target element 6 =" + index);
+    }
+}

en/codes/java/chapter_divide_and_conquer/build_tree.java

@@ -0,0 +1,51 @@
+/**
+ * File: build_tree.java
+ * Created Time: 2023-07-17
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_divide_and_conquer;
+
+import utils.*;
+import java.util.*;
+
+public class build_tree {
+    /* Build binary tree: Divide and conquer */
+    static TreeNode dfs(int[] preorder, Map<Integer, Integer> inorderMap, int i, int l, int r) {
+        // Terminate when subtree interval is empty
+        if (r - l < 0)
+            return null;
+        // Initialize root node
+        TreeNode root = new TreeNode(preorder[i]);
+        // Query m to divide left and right subtrees
+        int m = inorderMap.get(preorder[i]);
+        // Subproblem: build left subtree
+        root.left = dfs(preorder, inorderMap, i + 1, l, m - 1);
+        // Subproblem: build right subtree
+        root.right = dfs(preorder, inorderMap, i + 1 + m - l, m + 1, r);
+        // Return root node
+        return root;
+    }
+
+    /* Build binary tree */
+    static TreeNode buildTree(int[] preorder, int[] inorder) {
+        // Initialize hash table, storing in-order elements to indices mapping
+        Map<Integer, Integer> inorderMap = new HashMap<>();
+        for (int i = 0; i < inorder.length; i++) {
+            inorderMap.put(inorder[i], i);
+        }
+        TreeNode root = dfs(preorder, inorderMap, 0, 0, inorder.length - 1);
+        return root;
+    }
+
+    public static void main(String[] args) {
+        int[] preorder = { 3, 9, 2, 1, 7 };
+        int[] inorder = { 9, 3, 1, 2, 7 };
+        System.out.println("Pre-order traversal = " + Arrays.toString(preorder));
+        System.out.println("In-order traversal = " + Arrays.toString(inorder));
+
+        TreeNode root = buildTree(preorder, inorder);
+        System.out.println("The built binary tree is:");
+        PrintUtil.printTree(root);
+    }
+}

en/codes/java/chapter_divide_and_conquer/hanota.java

@@ -0,0 +1,59 @@
+/**
+ * File: hanota.java
+ * Created Time: 2023-07-17
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_divide_and_conquer;
+
+import java.util.*;
+
+public class hanota {
+    /* Move a disc */
+    static void move(List<Integer> src, List<Integer> tar) {
+        // Take out a disc from the top of src
+        Integer pan = src.remove(src.size() - 1);
+        // Place the disc on top of tar
+        tar.add(pan);
+    }
+
+    /* Solve the Tower of Hanoi problem f(i) */
+    static void dfs(int i, List<Integer> src, List<Integer> buf, List<Integer> tar) {
+        // If only one disc remains on src, move it to tar
+        if (i == 1) {
+            move(src, tar);
+            return;
+        }
+        // Subproblem f(i-1): move the top i-1 discs from src with the help of tar to buf
+        dfs(i - 1, src, tar, buf);
+        // Subproblem f(1): move the remaining one disc from src to tar
+        move(src, tar);
+        // Subproblem f(i-1): move the top i-1 discs from buf with the help of src to tar
+        dfs(i - 1, buf, src, tar);
+    }
+
+    /* Solve the Tower of Hanoi problem */
+    static void solveHanota(List<Integer> A, List<Integer> B, List<Integer> C) {
+        int n = A.size();
+        // Move the top n discs from A with the help of B to C
+        dfs(n, A, B, C);
+    }
+
+    public static void main(String[] args) {
+        // The tail of the list is the top of the pillar
+        List<Integer> A = new ArrayList<>(Arrays.asList(5, 4, 3, 2, 1));
+        List<Integer> B = new ArrayList<>();
+        List<Integer> C = new ArrayList<>();
+        System.out.println("Initial state:");
+        System.out.println("A = " + A);
+        System.out.println("B = " + B);
+        System.out.println("C = " + C);
+
+        solveHanota(A, B, C);
+
+        System.out.println("After the discs are moved:");
+        System.out.println("A = " + A);
+        System.out.println("B = " + B);
+        System.out.println("C = " + C);
+    }
+}

en/codes/java/chapter_dynamic_programming/climbing_stairs_backtrack.java

@@ -0,0 +1,44 @@
+/**
+ * File: climbing_stairs_backtrack.java
+ * Created Time: 2023-06-30
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_dynamic_programming;
+
+import java.util.*;
+
+public class climbing_stairs_backtrack {
+    /* Backtracking */
+    public static void backtrack(List<Integer> choices, int state, int n, List<Integer> res) {
+        // When climbing to the nth step, add 1 to the number of solutions
+        if (state == n)
+            res.set(0, res.get(0) + 1);
+        // Traverse all choices
+        for (Integer choice : choices) {
+            // Pruning: do not allow climbing beyond the nth step
+            if (state + choice > n)
+                continue;
+            // Attempt: make a choice, update the state
+            backtrack(choices, state + choice, n, res);
+            // Retract
+        }
+    }
+
+    /* Climbing stairs: Backtracking */
+    public static int climbingStairsBacktrack(int n) {
+        List<Integer> choices = Arrays.asList(1, 2); // Can choose to climb up 1 step or 2 steps
+        int state = 0; // Start climbing from the 0th step
+        List<Integer> res = new ArrayList<>();
+        res.add(0); // Use res[0] to record the number of solutions
+        backtrack(choices, state, n, res);
+        return res.get(0);
+    }
+
+    public static void main(String[] args) {
+        int n = 9;
+
+        int res = climbingStairsBacktrack(n);
+        System.out.println(String.format("There are %d solutions to climb %d stairs", n, res));
+    }
+}

en/codes/java/chapter_dynamic_programming/climbing_stairs_constraint_dp.java

@@ -0,0 +1,36 @@
+/**
+ * File: climbing_stairs_constraint_dp.java
+ * Created Time: 2023-07-01
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_dynamic_programming;
+
+public class climbing_stairs_constraint_dp {
+    /* Constrained climbing stairs: Dynamic programming */
+    static int climbingStairsConstraintDP(int n) {
+        if (n == 1 || n == 2) {
+            return 1;
+        }
+        // Initialize dp table, used to store subproblem solutions
+        int[][] dp = new int[n + 1][3];
+        // Initial state: preset the smallest subproblem solution
+        dp[1][1] = 1;
+        dp[1][2] = 0;
+        dp[2][1] = 0;
+        dp[2][2] = 1;
+        // State transition: gradually solve larger subproblems from smaller ones
+        for (int i = 3; i <= n; i++) {
+            dp[i][1] = dp[i - 1][2];
+            dp[i][2] = dp[i - 2][1] + dp[i - 2][2];
+        }
+        return dp[n][1] + dp[n][2];
+    }
+
+    public static void main(String[] args) {
+        int n = 9;
+
+        int res = climbingStairsConstraintDP(n);
+        System.out.println(String.format("There are %d solutions to climb %d stairs", n, res));
+    }
+}

en/codes/java/chapter_dynamic_programming/climbing_stairs_dfs.java

@@ -0,0 +1,31 @@
+/**
+ * File: climbing_stairs_dfs.java
+ * Created Time: 2023-06-30
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_dynamic_programming;
+
+public class climbing_stairs_dfs {
+    /* Search */
+    public static int dfs(int i) {
+        // Known dp[1] and dp[2], return them
+        if (i == 1 || i == 2)
+            return i;
+        // dp[i] = dp[i-1] + dp[i-2]
+        int count = dfs(i - 1) + dfs(i - 2);
+        return count;
+    }
+
+    /* Climbing stairs: Search */
+    public static int climbingStairsDFS(int n) {
+        return dfs(n);
+    }
+
+    public static void main(String[] args) {
+        int n = 9;
+
+        int res = climbingStairsDFS(n);
+        System.out.println(String.format("There are %d solutions to climb %d stairs", n, res));
+    }
+}

en/codes/java/chapter_dynamic_programming/climbing_stairs_dfs_mem.java

@@ -0,0 +1,41 @@
+/**
+ * File: climbing_stairs_dfs_mem.java
+ * Created Time: 2023-06-30
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_dynamic_programming;
+
+import java.util.Arrays;
+
+public class climbing_stairs_dfs_mem {
+    /* Memoized search */
+    public static int dfs(int i, int[] mem) {
+        // Known dp[1] and dp[2], return them
+        if (i == 1 || i == 2)
+            return i;
+        // If there is a record for dp[i], return it
+        if (mem[i] != -1)
+            return mem[i];
+        // dp[i] = dp[i-1] + dp[i-2]
+        int count = dfs(i - 1, mem) + dfs(i - 2, mem);
+        // Record dp[i]
+        mem[i] = count;
+        return count;
+    }
+
+    /* Climbing stairs: Memoized search */
+    public static int climbingStairsDFSMem(int n) {
+        // mem[i] records the total number of solutions for climbing to the ith step, -1 means no record
+        int[] mem = new int[n + 1];
+        Arrays.fill(mem, -1);
+        return dfs(n, mem);
+    }
+
+    public static void main(String[] args) {
+        int n = 9;
+
+        int res = climbingStairsDFSMem(n);
+        System.out.println(String.format("There are %d solutions to climb %d stairs", n, res));
+    }
+}

en/codes/java/chapter_dynamic_programming/climbing_stairs_dp.java

@@ -0,0 +1,48 @@
+/**
+ * File: climbing_stairs_dp.java
+ * Created Time: 2023-06-30
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_dynamic_programming;
+
+public class climbing_stairs_dp {
+    /* Climbing stairs: Dynamic programming */
+    public static int climbingStairsDP(int n) {
+        if (n == 1 || n == 2)
+            return n;
+        // Initialize dp table, used to store subproblem solutions
+        int[] dp = new int[n + 1];
+        // Initial state: preset the smallest subproblem solution
+        dp[1] = 1;
+        dp[2] = 2;
+        // State transition: gradually solve larger subproblems from smaller ones
+        for (int i = 3; i <= n; i++) {
+            dp[i] = dp[i - 1] + dp[i - 2];
+        }
+        return dp[n];
+    }
+
+    /* Climbing stairs: Space-optimized dynamic programming */
+    public static int climbingStairsDPComp(int n) {
+        if (n == 1 || n == 2)
+            return n;
+        int a = 1, b = 2;
+        for (int i = 3; i <= n; i++) {
+            int tmp = b;
+            b = a + b;
+            a = tmp;
+        }
+        return b;
+    }
+
+    public static void main(String[] args) {
+        int n = 9;
+
+        int res = climbingStairsDP(n);
+        System.out.println(String.format("There are %d solutions to climb %d stairs", n, res));
+
+        res = climbingStairsDPComp(n);
+        System.out.println(String.format("There are %d solutions to climb %d stairs", n, res));
+    }
+}

en/codes/java/chapter_dynamic_programming/coin_change.java

@@ -0,0 +1,72 @@
+/**
+ * File: coin_change.java
+ * Created Time: 2023-07-11
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_dynamic_programming;
+
+import java.util.Arrays;
+
+public class coin_change {
+    /* Coin change: Dynamic programming */
+    static int coinChangeDP(int[] coins, int amt) {
+        int n = coins.length;
+        int MAX = amt + 1;
+        // Initialize dp table
+        int[][] dp = new int[n + 1][amt + 1];
+        // State transition: first row and first column
+        for (int a = 1; a <= amt; a++) {
+            dp[0][a] = MAX;
+        }
+        // State transition: the rest of the rows and columns
+        for (int i = 1; i <= n; i++) {
+            for (int a = 1; a <= amt; a++) {
+                if (coins[i - 1] > a) {
+                    // If exceeding the target amount, do not choose coin i
+                    dp[i][a] = dp[i - 1][a];
+                } else {
+                    // The smaller value between not choosing and choosing coin i
+                    dp[i][a] = Math.min(dp[i - 1][a], dp[i][a - coins[i - 1]] + 1);
+                }
+            }
+        }
+        return dp[n][amt] != MAX ? dp[n][amt] : -1;
+    }
+
+    /* Coin change: Space-optimized dynamic programming */
+    static int coinChangeDPComp(int[] coins, int amt) {
+        int n = coins.length;
+        int MAX = amt + 1;
+        // Initialize dp table
+        int[] dp = new int[amt + 1];
+        Arrays.fill(dp, MAX);
+        dp[0] = 0;
+        // State transition
+        for (int i = 1; i <= n; i++) {
+            for (int a = 1; a <= amt; a++) {
+                if (coins[i - 1] > a) {
+                    // If exceeding the target amount, do not choose coin i
+                    dp[a] = dp[a];
+                } else {
+                    // The smaller value between not choosing and choosing coin i
+                    dp[a] = Math.min(dp[a], dp[a - coins[i - 1]] + 1);
+                }
+            }
+        }
+        return dp[amt] != MAX ? dp[amt] : -1;
+    }
+
+    public static void main(String[] args) {
+        int[] coins = { 1, 2, 5 };
+        int amt = 4;
+
+        // Dynamic programming
+        int res = coinChangeDP(coins, amt);
+        System.out.println("The minimum number of coins required to make up the target amount is " + res);
+
+        // Space-optimized dynamic programming
+        res = coinChangeDPComp(coins, amt);
+        System.out.println("The minimum number of coins required to make up the target amount is " + res);
+    }
+}

en/codes/java/chapter_dynamic_programming/coin_change_ii.java

@@ -0,0 +1,67 @@
+/**
+ * File: coin_change_ii.java
+ * Created Time: 2023-07-11
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_dynamic_programming;
+
+public class coin_change_ii {
+    /* Coin change II: Dynamic programming */
+    static int coinChangeIIDP(int[] coins, int amt) {
+        int n = coins.length;
+        // Initialize dp table
+        int[][] dp = new int[n + 1][amt + 1];
+        // Initialize first column
+        for (int i = 0; i <= n; i++) {
+            dp[i][0] = 1;
+        }
+        // State transition
+        for (int i = 1; i <= n; i++) {
+            for (int a = 1; a <= amt; a++) {
+                if (coins[i - 1] > a) {
+                    // If exceeding the target amount, do not choose coin i
+                    dp[i][a] = dp[i - 1][a];
+                } else {
+                    // The sum of the two options of not choosing and choosing coin i
+                    dp[i][a] = dp[i - 1][a] + dp[i][a - coins[i - 1]];
+                }
+            }
+        }
+        return dp[n][amt];
+    }
+
+    /* Coin change II: Space-optimized dynamic programming */
+    static int coinChangeIIDPComp(int[] coins, int amt) {
+        int n = coins.length;
+        // Initialize dp table
+        int[] dp = new int[amt + 1];
+        dp[0] = 1;
+        // State transition
+        for (int i = 1; i <= n; i++) {
+            for (int a = 1; a <= amt; a++) {
+                if (coins[i - 1] > a) {
+                    // If exceeding the target amount, do not choose coin i
+                    dp[a] = dp[a];
+                } else {
+                    // The sum of the two options of not choosing and choosing coin i
+                    dp[a] = dp[a] + dp[a - coins[i - 1]];
+                }
+            }
+        }
+        return dp[amt];
+    }
+
+    public static void main(String[] args) {
+        int[] coins = { 1, 2, 5 };
+        int amt = 5;
+
+        // Dynamic programming
+        int res = coinChangeIIDP(coins, amt);
+        System.out.println("The number of coin combinations to make up the target amount is " + res);
+
+        // Space-optimized dynamic programming
+        res = coinChangeIIDPComp(coins, amt);
+        System.out.println("The number of coin combinations to make up the target amount is " + res);
+    }
+}

en/codes/java/chapter_dynamic_programming/edit_distance.java

@@ -0,0 +1,139 @@
+/**
+ * File: edit_distance.java
+ * Created Time: 2023-07-13
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_dynamic_programming;
+
+import java.util.Arrays;
+
+public class edit_distance {
+    /* Edit distance: Brute force search */
+    static int editDistanceDFS(String s, String t, int i, int j) {
+        // If both s and t are empty, return 0
+        if (i == 0 && j == 0)
+            return 0;
+        // If s is empty, return the length of t
+        if (i == 0)
+            return j;
+        // If t is empty, return the length of s
+        if (j == 0)
+            return i;
+        // If the two characters are equal, skip these two characters
+        if (s.charAt(i - 1) == t.charAt(j - 1))
+            return editDistanceDFS(s, t, i - 1, j - 1);
+        // The minimum number of edits = the minimum number of edits from three operations (insert, remove, replace) + 1
+        int insert = editDistanceDFS(s, t, i, j - 1);
+        int delete = editDistanceDFS(s, t, i - 1, j);
+        int replace = editDistanceDFS(s, t, i - 1, j - 1);
+        // Return the minimum number of edits
+        return Math.min(Math.min(insert, delete), replace) + 1;
+    }
+
+    /* Edit distance: Memoized search */
+    static int editDistanceDFSMem(String s, String t, int[][] mem, int i, int j) {
+        // If both s and t are empty, return 0
+        if (i == 0 && j == 0)
+            return 0;
+        // If s is empty, return the length of t
+        if (i == 0)
+            return j;
+        // If t is empty, return the length of s
+        if (j == 0)
+            return i;
+        // If there is a record, return it
+        if (mem[i][j] != -1)
+            return mem[i][j];
+        // If the two characters are equal, skip these two characters
+        if (s.charAt(i - 1) == t.charAt(j - 1))
+            return editDistanceDFSMem(s, t, mem, i - 1, j - 1);
+        // The minimum number of edits = the minimum number of edits from three operations (insert, remove, replace) + 1
+        int insert = editDistanceDFSMem(s, t, mem, i, j - 1);
+        int delete = editDistanceDFSMem(s, t, mem, i - 1, j);
+        int replace = editDistanceDFSMem(s, t, mem, i - 1, j - 1);
+        // Record and return the minimum number of edits
+        mem[i][j] = Math.min(Math.min(insert, delete), replace) + 1;
+        return mem[i][j];
+    }
+
+    /* Edit distance: Dynamic programming */
+    static int editDistanceDP(String s, String t) {
+        int n = s.length(), m = t.length();
+        int[][] dp = new int[n + 1][m + 1];
+        // State transition: first row and first column
+        for (int i = 1; i <= n; i++) {
+            dp[i][0] = i;
+        }
+        for (int j = 1; j <= m; j++) {
+            dp[0][j] = j;
+        }
+        // State transition: the rest of the rows and columns
+        for (int i = 1; i <= n; i++) {
+            for (int j = 1; j <= m; j++) {
+                if (s.charAt(i - 1) == t.charAt(j - 1)) {
+                    // If the two characters are equal, skip these two characters
+                    dp[i][j] = dp[i - 1][j - 1];
+                } else {
+                    // The minimum number of edits = the minimum number of edits from three operations (insert, remove, replace) + 1
+                    dp[i][j] = Math.min(Math.min(dp[i][j - 1], dp[i - 1][j]), dp[i - 1][j - 1]) + 1;
+                }
+            }
+        }
+        return dp[n][m];
+    }
+
+    /* Edit distance: Space-optimized dynamic programming */
+    static int editDistanceDPComp(String s, String t) {
+        int n = s.length(), m = t.length();
+        int[] dp = new int[m + 1];
+        // State transition: first row
+        for (int j = 1; j <= m; j++) {
+            dp[j] = j;
+        }
+        // State transition: the rest of the rows
+        for (int i = 1; i <= n; i++) {
+            // State transition: first column
+            int leftup = dp[0]; // Temporarily store dp[i-1, j-1]
+            dp[0] = i;
+            // State transition: the rest of the columns
+            for (int j = 1; j <= m; j++) {
+                int temp = dp[j];
+                if (s.charAt(i - 1) == t.charAt(j - 1)) {
+                    // If the two characters are equal, skip these two characters
+                    dp[j] = leftup;
+                } else {
+                    // The minimum number of edits = the minimum number of edits from three operations (insert, remove, replace) + 1
+                    dp[j] = Math.min(Math.min(dp[j - 1], dp[j]), leftup) + 1;
+                }
+                leftup = temp; // Update for the next round of dp[i-1, j-1]
+            }
+        }
+        return dp[m];
+    }
+
+    public static void main(String[] args) {
+        String s = "bag";
+        String t = "pack";
+        int n = s.length(), m = t.length();
+
+        // Brute force search
+        int res = editDistanceDFS(s, t, n, m);
+        System.out.println("Changing " + s + " to " + t + " requires a minimum of " + res + " edits");
+
+        // Memoized search
+        int[][] mem = new int[n + 1][m + 1];
+        for (int[] row : mem)
+            Arrays.fill(row, -1);
+        res = editDistanceDFSMem(s, t, mem, n, m);
+        System.out.println("Changing " + s + " to " + t + " requires a minimum of " + res + " edits");
+
+        // Dynamic programming
+        res = editDistanceDP(s, t);
+        System.out.println("Changing " + s + " to " + t + " requires a minimum of " + res + " edits");
+
+        // Space-optimized dynamic programming
+        res = editDistanceDPComp(s, t);
+        System.out.println("Changing " + s + " to " + t + " requires a minimum of " + res + " edits");
+    }
+}

en/codes/java/chapter_dynamic_programming/knapsack.java

@@ -0,0 +1,116 @@
+/**
+ * File: knapsack.java
+ * Created Time: 2023-07-10
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_dynamic_programming;
+
+import java.util.Arrays;
+
+public class knapsack {
+
+    /* 0-1 Knapsack: Brute force search */
+    static int knapsackDFS(int[] wgt, int[] val, int i, int c) {
+        // If all items have been chosen or the knapsack has no remaining capacity, return value 0
+        if (i == 0 || c == 0) {
+            return 0;
+        }
+        // If exceeding the knapsack capacity, can only choose not to put it in the knapsack
+        if (wgt[i - 1] > c) {
+            return knapsackDFS(wgt, val, i - 1, c);
+        }
+        // Calculate the maximum value of not putting in and putting in item i
+        int no = knapsackDFS(wgt, val, i - 1, c);
+        int yes = knapsackDFS(wgt, val, i - 1, c - wgt[i - 1]) + val[i - 1];
+        // Return the greater value of the two options
+        return Math.max(no, yes);
+    }
+
+    /* 0-1 Knapsack: Memoized search */
+    static int knapsackDFSMem(int[] wgt, int[] val, int[][] mem, int i, int c) {
+        // If all items have been chosen or the knapsack has no remaining capacity, return value 0
+        if (i == 0 || c == 0) {
+            return 0;
+        }
+        // If there is a record, return it
+        if (mem[i][c] != -1) {
+            return mem[i][c];
+        }
+        // If exceeding the knapsack capacity, can only choose not to put it in the knapsack
+        if (wgt[i - 1] > c) {
+            return knapsackDFSMem(wgt, val, mem, i - 1, c);
+        }
+        // Calculate the maximum value of not putting in and putting in item i
+        int no = knapsackDFSMem(wgt, val, mem, i - 1, c);
+        int yes = knapsackDFSMem(wgt, val, mem, i - 1, c - wgt[i - 1]) + val[i - 1];
+        // Record and return the greater value of the two options
+        mem[i][c] = Math.max(no, yes);
+        return mem[i][c];
+    }
+
+    /* 0-1 Knapsack: Dynamic programming */
+    static int knapsackDP(int[] wgt, int[] val, int cap) {
+        int n = wgt.length;
+        // Initialize dp table
+        int[][] dp = new int[n + 1][cap + 1];
+        // State transition
+        for (int i = 1; i <= n; i++) {
+            for (int c = 1; c <= cap; c++) {
+                if (wgt[i - 1] > c) {
+                    // If exceeding the knapsack capacity, do not choose item i
+                    dp[i][c] = dp[i - 1][c];
+                } else {
+                    // The greater value between not choosing and choosing item i
+                    dp[i][c] = Math.max(dp[i - 1][c], dp[i - 1][c - wgt[i - 1]] + val[i - 1]);
+                }
+            }
+        }
+        return dp[n][cap];
+    }
+
+    /* 0-1 Knapsack: Space-optimized dynamic programming */
+    static int knapsackDPComp(int[] wgt, int[] val, int cap) {
+        int n = wgt.length;
+        // Initialize dp table
+        int[] dp = new int[cap + 1];
+        // State transition
+        for (int i = 1; i <= n; i++) {
+            // Traverse in reverse order
+            for (int c = cap; c >= 1; c--) {
+                if (wgt[i - 1] <= c) {
+                    // The greater value between not choosing and choosing item i
+                    dp[c] = Math.max(dp[c], dp[c - wgt[i - 1]] + val[i - 1]);
+                }
+            }
+        }
+        return dp[cap];
+    }
+
+    public static void main(String[] args) {
+        int[] wgt = { 10, 20, 30, 40, 50 };
+        int[] val = { 50, 120, 150, 210, 240 };
+        int cap = 50;
+        int n = wgt.length;
+
+        // Brute force search
+        int res = knapsackDFS(wgt, val, n, cap);
+        System.out.println("The maximum value within the bag capacity is " + res);
+
+        // Memoized search
+        int[][] mem = new int[n + 1][cap + 1];
+        for (int[] row : mem) {
+            Arrays.fill(row, -1);
+        }
+        res = knapsackDFSMem(wgt, val, mem, n, cap);
+        System.out.println("The maximum value within the bag capacity is " + res);
+
+        // Dynamic programming
+        res = knapsackDP(wgt, val, cap);
+        System.out.println("The maximum value within the bag capacity is " + res);
+
+        // Space-optimized dynamic programming
+        res = knapsackDPComp(wgt, val, cap);
+        System.out.println("The maximum value within the bag capacity is " + res);
+    }
+}

en/codes/java/chapter_dynamic_programming/min_cost_climbing_stairs_dp.java

@@ -0,0 +1,53 @@
+/**
+ * File: min_cost_climbing_stairs_dp.java
+ * Created Time: 2023-06-30
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_dynamic_programming;
+
+import java.util.Arrays;
+
+public class min_cost_climbing_stairs_dp {
+    /* Climbing stairs with minimum cost: Dynamic programming */
+    public static int minCostClimbingStairsDP(int[] cost) {
+        int n = cost.length - 1;
+        if (n == 1 || n == 2)
+            return cost[n];
+        // Initialize dp table, used to store subproblem solutions
+        int[] dp = new int[n + 1];
+        // Initial state: preset the smallest subproblem solution
+        dp[1] = cost[1];
+        dp[2] = cost[2];
+        // State transition: gradually solve larger subproblems from smaller ones
+        for (int i = 3; i <= n; i++) {
+            dp[i] = Math.min(dp[i - 1], dp[i - 2]) + cost[i];
+        }
+        return dp[n];
+    }
+
+    /* Climbing stairs with minimum cost: Space-optimized dynamic programming */
+    public static int minCostClimbingStairsDPComp(int[] cost) {
+        int n = cost.length - 1;
+        if (n == 1 || n == 2)
+            return cost[n];
+        int a = cost[1], b = cost[2];
+        for (int i = 3; i <= n; i++) {
+            int tmp = b;
+            b = Math.min(a, tmp) + cost[i];
+            a = tmp;
+        }
+        return b;
+    }
+
+    public static void main(String[] args) {
+        int[] cost = { 0, 1, 10, 1, 1, 1, 10, 1, 1, 10, 1 };
+        System.out.println(String.format("Input the cost list for stairs as %s", Arrays.toString(cost)));
+
+        int res = minCostClimbingStairsDP(cost);
+        System.out.println(String.format("Minimum cost to climb the stairs %d", res));
+
+        res = minCostClimbingStairsDPComp(cost);
+        System.out.println(String.format("Minimum cost to climb the stairs %d", res));
+    }
+}

en/codes/java/chapter_dynamic_programming/min_path_sum.java

@@ -0,0 +1,125 @@
+/**
+ * File: min_path_sum.java
+ * Created Time: 2023-07-10
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_dynamic_programming;
+
+import java.util.Arrays;
+
+public class min_path_sum {
+    /* Minimum path sum: Brute force search */
+    static int minPathSumDFS(int[][] grid, int i, int j) {
+        // If it's the top-left cell, terminate the search
+        if (i == 0 && j == 0) {
+            return grid[0][0];
+        }
+        // If the row or column index is out of bounds, return a +∞ cost
+        if (i < 0 || j < 0) {
+            return Integer.MAX_VALUE;
+        }
+        // Calculate the minimum path cost from the top-left to (i-1, j) and (i, j-1)
+        int up = minPathSumDFS(grid, i - 1, j);
+        int left = minPathSumDFS(grid, i, j - 1);
+        // Return the minimum path cost from the top-left to (i, j)
+        return Math.min(left, up) + grid[i][j];
+    }
+
+    /* Minimum path sum: Memoized search */
+    static int minPathSumDFSMem(int[][] grid, int[][] mem, int i, int j) {
+        // If it's the top-left cell, terminate the search
+        if (i == 0 && j == 0) {
+            return grid[0][0];
+        }
+        // If the row or column index is out of bounds, return a +∞ cost
+        if (i < 0 || j < 0) {
+            return Integer.MAX_VALUE;
+        }
+        // If there is a record, return it
+        if (mem[i][j] != -1) {
+            return mem[i][j];
+        }
+        // The minimum path cost from the left and top cells
+        int up = minPathSumDFSMem(grid, mem, i - 1, j);
+        int left = minPathSumDFSMem(grid, mem, i, j - 1);
+        // Record and return the minimum path cost from the top-left to (i, j)
+        mem[i][j] = Math.min(left, up) + grid[i][j];
+        return mem[i][j];
+    }
+
+    /* Minimum path sum: Dynamic programming */
+    static int minPathSumDP(int[][] grid) {
+        int n = grid.length, m = grid[0].length;
+        // Initialize dp table
+        int[][] dp = new int[n][m];
+        dp[0][0] = grid[0][0];
+        // State transition: first row
+        for (int j = 1; j < m; j++) {
+            dp[0][j] = dp[0][j - 1] + grid[0][j];
+        }
+        // State transition: first column
+        for (int i = 1; i < n; i++) {
+            dp[i][0] = dp[i - 1][0] + grid[i][0];
+        }
+        // State transition: the rest of the rows and columns
+        for (int i = 1; i < n; i++) {
+            for (int j = 1; j < m; j++) {
+                dp[i][j] = Math.min(dp[i][j - 1], dp[i - 1][j]) + grid[i][j];
+            }
+        }
+        return dp[n - 1][m - 1];
+    }
+
+    /* Minimum path sum: Space-optimized dynamic programming */
+    static int minPathSumDPComp(int[][] grid) {
+        int n = grid.length, m = grid[0].length;
+        // Initialize dp table
+        int[] dp = new int[m];
+        // State transition: first row
+        dp[0] = grid[0][0];
+        for (int j = 1; j < m; j++) {
+            dp[j] = dp[j - 1] + grid[0][j];
+        }
+        // State transition: the rest of the rows
+        for (int i = 1; i < n; i++) {
+            // State transition: first column
+            dp[0] = dp[0] + grid[i][0];
+            // State transition: the rest of the columns
+            for (int j = 1; j < m; j++) {
+                dp[j] = Math.min(dp[j - 1], dp[j]) + grid[i][j];
+            }
+        }
+        return dp[m - 1];
+    }
+
+    public static void main(String[] args) {
+        int[][] grid = {
+                { 1, 3, 1, 5 },
+                { 2, 2, 4, 2 },
+                { 5, 3, 2, 1 },
+                { 4, 3, 5, 2 }
+        };
+        int n = grid.length, m = grid[0].length;
+
+        // Brute force search
+        int res = minPathSumDFS(grid, n - 1, m - 1);
+        System.out.println("The minimum path sum from the top left corner to the bottom right corner is " + res);
+
+        // Memoized search
+        int[][] mem = new int[n][m];
+        for (int[] row : mem) {
+            Arrays.fill(row, -1);
+        }
+        res = minPathSumDFSMem(grid, mem, n - 1, m - 1);
+        System.out.println("The minimum path sum from the top left corner to the bottom right corner is " + res);
+
+        // Dynamic programming
+        res = minPathSumDP(grid);
+        System.out.println("The minimum path sum from the top left corner to the bottom right corner is " + res);
+
+        // Space-optimized dynamic programming
+        res = minPathSumDPComp(grid);
+        System.out.println("The minimum path sum from the top left corner to the bottom right corner is " + res);
+    }
+}

en/codes/java/chapter_dynamic_programming/unbounded_knapsack.java

@@ -0,0 +1,63 @@
+/**
+ * File: unbounded_knapsack.java
+ * Created Time: 2023-07-11
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_dynamic_programming;
+
+public class unbounded_knapsack {
+    /* Complete knapsack: Dynamic programming */
+    static int unboundedKnapsackDP(int[] wgt, int[] val, int cap) {
+        int n = wgt.length;
+        // Initialize dp table
+        int[][] dp = new int[n + 1][cap + 1];
+        // State transition
+        for (int i = 1; i <= n; i++) {
+            for (int c = 1; c <= cap; c++) {
+                if (wgt[i - 1] > c) {
+                    // If exceeding the knapsack capacity, do not choose item i
+                    dp[i][c] = dp[i - 1][c];
+                } else {
+                    // The greater value between not choosing and choosing item i
+                    dp[i][c] = Math.max(dp[i - 1][c], dp[i][c - wgt[i - 1]] + val[i - 1]);
+                }
+            }
+        }
+        return dp[n][cap];
+    }
+
+    /* Complete knapsack: Space-optimized dynamic programming */
+    static int unboundedKnapsackDPComp(int[] wgt, int[] val, int cap) {
+        int n = wgt.length;
+        // Initialize dp table
+        int[] dp = new int[cap + 1];
+        // State transition
+        for (int i = 1; i <= n; i++) {
+            for (int c = 1; c <= cap; c++) {
+                if (wgt[i - 1] > c) {
+                    // If exceeding the knapsack capacity, do not choose item i
+                    dp[c] = dp[c];
+                } else {
+                    // The greater value between not choosing and choosing item i
+                    dp[c] = Math.max(dp[c], dp[c - wgt[i - 1]] + val[i - 1]);
+                }
+            }
+        }
+        return dp[cap];
+    }
+
+    public static void main(String[] args) {
+        int[] wgt = { 1, 2, 3 };
+        int[] val = { 5, 11, 15 };
+        int cap = 4;
+
+        // Dynamic programming
+        int res = unboundedKnapsackDP(wgt, val, cap);
+        System.out.println("The maximum value within the bag capacity is " + res);
+
+        // Space-optimized dynamic programming
+        res = unboundedKnapsackDPComp(wgt, val, cap);
+        System.out.println("The maximum value within the bag capacity is " + res);
+    }
+}

en/codes/java/chapter_graph/graph_adjacency_list.java

@@ -0,0 +1,117 @@
+/**
+ * File: graph_adjacency_list.java
+ * Created Time: 2023-01-26
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_graph;
+
+import java.util.*;
+import utils.*;
+
+/* Undirected graph class based on adjacency list */
+class GraphAdjList {
+    // Adjacency list, key: vertex, value: all adjacent vertices of that vertex
+    Map<Vertex, List<Vertex>> adjList;
+
+    /* Constructor */
+    public GraphAdjList(Vertex[][] edges) {
+        this.adjList = new HashMap<>();
+        // Add all vertices and edges
+        for (Vertex[] edge : edges) {
+            addVertex(edge[0]);
+            addVertex(edge[1]);
+            addEdge(edge[0], edge[1]);
+        }
+    }
+
+    /* Get the number of vertices */
+    public int size() {
+        return adjList.size();
+    }
+
+    /* Add edge */
+    public void addEdge(Vertex vet1, Vertex vet2) {
+        if (!adjList.containsKey(vet1) || !adjList.containsKey(vet2) || vet1 == vet2)
+            throw new IllegalArgumentException();
+        // Add edge vet1 - vet2
+        adjList.get(vet1).add(vet2);
+        adjList.get(vet2).add(vet1);
+    }
+
+    /* Remove edge */
+    public void removeEdge(Vertex vet1, Vertex vet2) {
+        if (!adjList.containsKey(vet1) || !adjList.containsKey(vet2) || vet1 == vet2)
+            throw new IllegalArgumentException();
+        // Remove edge vet1 - vet2
+        adjList.get(vet1).remove(vet2);
+        adjList.get(vet2).remove(vet1);
+    }
+
+    /* Add vertex */
+    public void addVertex(Vertex vet) {
+        if (adjList.containsKey(vet))
+            return;
+        // Add a new linked list to the adjacency list
+        adjList.put(vet, new ArrayList<>());
+    }
+
+    /* Remove vertex */
+    public void removeVertex(Vertex vet) {
+        if (!adjList.containsKey(vet))
+            throw new IllegalArgumentException();
+        // Remove the vertex vet's corresponding linked list from the adjacency list
+        adjList.remove(vet);
+        // Traverse other vertices' linked lists, removing all edges containing vet
+        for (List<Vertex> list : adjList.values()) {
+            list.remove(vet);
+        }
+    }
+
+    /* Print the adjacency list */
+    public void print() {
+        System.out.println("Adjacency list =");
+        for (Map.Entry<Vertex, List<Vertex>> pair : adjList.entrySet()) {
+            List<Integer> tmp = new ArrayList<>();
+            for (Vertex vertex : pair.getValue())
+                tmp.add(vertex.val);
+            System.out.println(pair.getKey().val + ": " + tmp + ",");
+        }
+    }
+}
+
+public class graph_adjacency_list {
+    public static void main(String[] args) {
+        /* Initialize undirected graph */
+        Vertex[] v = Vertex.valsToVets(new int[] { 1, 3, 2, 5, 4 });
+        Vertex[][] edges = { { v[0], v[1] }, { v[0], v[3] }, { v[1], v[2] },
+                             { v[2], v[3] }, { v[2], v[4] }, { v[3], v[4] } };
+        GraphAdjList graph = new GraphAdjList(edges);
+        System.out.println("\nAfter initialization, the graph is");
+        graph.print();
+
+        /* Add edge */
+        // Vertices 1, 2 i.e., v[0], v[2]
+        graph.addEdge(v[0], v[2]);
+        System.out.println("\nAfter adding edge 1-2, the graph is");
+        graph.print();
+
+        /* Remove edge */
+        // Vertices 1, 3 i.e., v[0], v[1]
+        graph.removeEdge(v[0], v[1]);
+        System.out.println("\nAfter removing edge 1-3, the graph is");
+        graph.print();
+
+        /* Add vertex */
+        Vertex v5 = new Vertex(6);
+        graph.addVertex(v5);
+        System.out.println("\nAfter adding vertex 6, the graph is");
+        graph.print();
+
+        /* Remove vertex */
+        // Vertex 3 i.e., v[1]
+        graph.removeVertex(v[1]);
+        System.out.println("\nAfter removing vertex 3, the graph is");
+        graph.print();
+    }
+}

en/codes/java/chapter_graph/graph_adjacency_matrix.java

@@ -0,0 +1,131 @@
+/**
+ * File: graph_adjacency_matrix.java
+ * Created Time: 2023-01-26
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_graph;
+
+import utils.*;
+import java.util.*;
+
+/* Undirected graph class based on adjacency matrix */
+class GraphAdjMat {
+    List<Integer> vertices; // Vertex list, elements represent "vertex value", index represents "vertex index"
+    List<List<Integer>> adjMat; // Adjacency matrix, row and column indices correspond to "vertex index"
+
+    /* Constructor */
+    public GraphAdjMat(int[] vertices, int[][] edges) {
+        this.vertices = new ArrayList<>();
+        this.adjMat = new ArrayList<>();
+        // Add vertex
+        for (int val : vertices) {
+            addVertex(val);
+        }
+        // Add edge
+        // Please note, edges elements represent vertex indices, corresponding to vertices elements indices
+        for (int[] e : edges) {
+            addEdge(e[0], e[1]);
+        }
+    }
+
+    /* Get the number of vertices */
+    public int size() {
+        return vertices.size();
+    }
+
+    /* Add vertex */
+    public void addVertex(int val) {
+        int n = size();
+        // Add new vertex value to the vertex list
+        vertices.add(val);
+        // Add a row to the adjacency matrix
+        List<Integer> newRow = new ArrayList<>(n);
+        for (int j = 0; j < n; j++) {
+            newRow.add(0);
+        }
+        adjMat.add(newRow);
+        // Add a column to the adjacency matrix
+        for (List<Integer> row : adjMat) {
+            row.add(0);
+        }
+    }
+
+    /* Remove vertex */
+    public void removeVertex(int index) {
+        if (index >= size())
+            throw new IndexOutOfBoundsException();
+        // Remove vertex at `index` from the vertex list
+        vertices.remove(index);
+        // Remove the row at `index` from the adjacency matrix
+        adjMat.remove(index);
+        // Remove the column at `index` from the adjacency matrix
+        for (List<Integer> row : adjMat) {
+            row.remove(index);
+        }
+    }
+
+    /* Add edge */
+    // Parameters i, j correspond to vertices element indices
+    public void addEdge(int i, int j) {
+        // Handle index out of bounds and equality
+        if (i < 0 || j < 0 || i >= size() || j >= size() || i == j)
+            throw new IndexOutOfBoundsException();
+        // In an undirected graph, the adjacency matrix is symmetric about the main diagonal, i.e., satisfies (i, j) == (j, i)
+        adjMat.get(i).set(j, 1);
+        adjMat.get(j).set(i, 1);
+    }
+
+    /* Remove edge */
+    // Parameters i, j correspond to vertices element indices
+    public void removeEdge(int i, int j) {
+        // Handle index out of bounds and equality
+        if (i < 0 || j < 0 || i >= size() || j >= size() || i == j)
+            throw new IndexOutOfBoundsException();
+        adjMat.get(i).set(j, 0);
+        adjMat.get(j).set(i, 0);
+    }
+
+    /* Print adjacency matrix */
+    public void print() {
+        System.out.print("Vertex list = ");
+        System.out.println(vertices);
+        System.out.println("Adjacency matrix =");
+        PrintUtil.printMatrix(adjMat);
+    }
+}
+
+public class graph_adjacency_matrix {
+    public static void main(String[] args) {
+        /* Initialize undirected graph */
+        // Please note, edges elements represent vertex indices, corresponding to vertices elements indices
+        int[] vertices = { 1, 3, 2, 5, 4 };
+        int[][] edges = { { 0, 1 }, { 0, 3 }, { 1, 2 }, { 2, 3 }, { 2, 4 }, { 3, 4 } };
+        GraphAdjMat graph = new GraphAdjMat(vertices, edges);
+        System.out.println("\nAfter initialization, the graph is");
+        graph.print();
+
+        /* Add edge */
+        // Indices of vertices 1, 2 are 0, 2 respectively
+        graph.addEdge(0, 2);
+        System.out.println("\nAfter adding edge 1-2, the graph is");
+        graph.print();
+
+        /* Remove edge */
+        // Indices of vertices 1, 3 are 0, 1 respectively
+        graph.removeEdge(0, 1);
+        System.out.println("\nAfter removing edge 1-3, the graph is");
+        graph.print();
+
+        /* Add vertex */
+        graph.addVertex(6);
+        System.out.println("\nAfter adding vertex 6, the graph is");
+        graph.print();
+
+        /* Remove vertex */
+        // Index of vertex 3 is 1
+        graph.removeVertex(1);
+        System.out.println("\nAfter removing vertex 3, the graph is");
+        graph.print();
+    }
+}

en/codes/java/chapter_graph/graph_bfs.java

@@ -0,0 +1,55 @@
+/**
+ * File: graph_bfs.java
+ * Created Time: 2023-02-12
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_graph;
+
+import java.util.*;
+import utils.*;
+
+public class graph_bfs {
+    /* Breadth-first traversal */
+    // Use adjacency list to represent the graph, to obtain all adjacent vertices of a specified vertex
+    static List<Vertex> graphBFS(GraphAdjList graph, Vertex startVet) {
+        // Vertex traversal sequence
+        List<Vertex> res = new ArrayList<>();
+        // Hash set, used to record visited vertices
+        Set<Vertex> visited = new HashSet<>();
+        visited.add(startVet);
+        // Queue used to implement BFS
+        Queue<Vertex> que = new LinkedList<>();
+        que.offer(startVet);
+        // Starting from vertex vet, loop until all vertices are visited
+        while (!que.isEmpty()) {
+            Vertex vet = que.poll(); // Dequeue the vertex at the head of the queue
+            res.add(vet);            // Record visited vertex
+            // Traverse all adjacent vertices of that vertex
+            for (Vertex adjVet : graph.adjList.get(vet)) {
+                if (visited.contains(adjVet))
+                    continue;        // Skip already visited vertices
+                que.offer(adjVet);   // Only enqueue unvisited vertices
+                visited.add(adjVet); // Mark the vertex as visited
+            }
+        }
+        // Return the vertex traversal sequence
+        return res;
+    }
+
+    public static void main(String[] args) {
+        /* Initialize undirected graph */
+        Vertex[] v = Vertex.valsToVets(new int[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 });
+        Vertex[][] edges = { { v[0], v[1] }, { v[0], v[3] }, { v[1], v[2] }, { v[1], v[4] },
+                             { v[2], v[5] }, { v[3], v[4] }, { v[3], v[6] }, { v[4], v[5] },
+                             { v[4], v[7] }, { v[5], v[8] }, { v[6], v[7] }, { v[7], v[8] } };
+        GraphAdjList graph = new GraphAdjList(edges);
+        System.out.println("\nAfter initialization, the graph is");
+        graph.print();
+
+        /* Breadth-first traversal */
+        List<Vertex> res = graphBFS(graph, v[0]);
+        System.out.println("\nBreadth-first traversal (BFS) vertex sequence is");
+        System.out.println(Vertex.vetsToVals(res));
+    }
+}

en/codes/java/chapter_graph/graph_dfs.java

@@ -0,0 +1,51 @@
+/**
+ * File: graph_dfs.java
+ * Created Time: 2023-02-12
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_graph;
+
+import java.util.*;
+import utils.*;
+
+public class graph_dfs {
+    /* Depth-first traversal helper function */
+    static void dfs(GraphAdjList graph, Set<Vertex> visited, List<Vertex> res, Vertex vet) {
+        res.add(vet);     // Record visited vertex
+        visited.add(vet); // Mark the vertex as visited
+        // Traverse all adjacent vertices of that vertex
+        for (Vertex adjVet : graph.adjList.get(vet)) {
+            if (visited.contains(adjVet))
+                continue; // Skip already visited vertices
+            // Recursively visit adjacent vertices
+            dfs(graph, visited, res, adjVet);
+        }
+    }
+
+    /* Depth-first traversal */
+    // Use adjacency list to represent the graph, to obtain all adjacent vertices of a specified vertex
+    static List<Vertex> graphDFS(GraphAdjList graph, Vertex startVet) {
+        // Vertex traversal sequence
+        List<Vertex> res = new ArrayList<>();
+        // Hash set, used to record visited vertices
+        Set<Vertex> visited = new HashSet<>();
+        dfs(graph, visited, res, startVet);
+        return res;
+    }
+
+    public static void main(String[] args) {
+        /* Initialize undirected graph */
+        Vertex[] v = Vertex.valsToVets(new int[] { 0, 1, 2, 3, 4, 5, 6 });
+        Vertex[][] edges = { { v[0], v[1] }, { v[0], v[3] }, { v[1], v[2] },
+                             { v[2], v[5] }, { v[4], v[5] }, { v[5], v[6] } };
+        GraphAdjList graph = new GraphAdjList(edges);
+        System.out.println("\nAfter initialization, the graph is");
+        graph.print();
+
+        /* Depth-first traversal */
+        List<Vertex> res = graphDFS(graph, v[0]);
+        System.out.println("\nDepth-first traversal (DFS) vertex sequence is");
+        System.out.println(Vertex.vetsToVals(res));
+    }
+}

en/codes/java/chapter_greedy/coin_change_greedy.java

@@ -0,0 +1,55 @@
+/**
+ * File: coin_change_greedy.java
+ * Created Time: 2023-07-20
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_greedy;
+
+import java.util.Arrays;
+
+public class coin_change_greedy {
+    /* Coin change: Greedy */
+    static int coinChangeGreedy(int[] coins, int amt) {
+        // Assume coins list is ordered
+        int i = coins.length - 1;
+        int count = 0;
+        // Loop for greedy selection until no remaining amount
+        while (amt > 0) {
+            // Find the smallest coin close to and less than the remaining amount
+            while (i > 0 && coins[i] > amt) {
+                i--;
+            }
+            // Choose coins[i]
+            amt -= coins[i];
+            count++;
+        }
+        // If no feasible solution is found, return -1
+        return amt == 0 ? count : -1;
+    }
+
+    public static void main(String[] args) {
+        // Greedy: can ensure finding a global optimal solution
+        int[] coins = { 1, 5, 10, 20, 50, 100 };
+        int amt = 186;
+        int res = coinChangeGreedy(coins, amt);
+        System.out.println("\ncoins = " + Arrays.toString(coins) + ", amt = " + amt);
+        System.out.println("The minimum number of coins required to make up " + amt + " is " + res);
+
+        // Greedy: cannot ensure finding a global optimal solution
+        coins = new int[] { 1, 20, 50 };
+        amt = 60;
+        res = coinChangeGreedy(coins, amt);
+        System.out.println("\ncoins = " + Arrays.toString(coins) + ", amt = " + amt);
+        System.out.println("The minimum number of coins required to make up " + amt + " is " + res);
+        System.out.println("In reality, the minimum number needed is 3, i.e., 20 + 20 + 20");
+
+        // Greedy: cannot ensure finding a global optimal solution
+        coins = new int[] { 1, 49, 50 };
+        amt = 98;
+        res = coinChangeGreedy(coins, amt);
+        System.out.println("\ncoins = " + Arrays.toString(coins) + ", amt = " + amt);
+        System.out.println("The minimum number of coins required to make up " + amt + " is " + res);
+        System.out.println("In reality, the minimum number needed is 2, i.e., 49 + 49");
+    }
+}

en/codes/java/chapter_greedy/fractional_knapsack.java

@@ -0,0 +1,59 @@
+/**
+ * File: fractional_knapsack.java
+ * Created Time: 2023-07-20
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_greedy;
+
+import java.util.Arrays;
+import java.util.Comparator;
+
+/* Item */
+class Item {
+    int w; // Item weight
+    int v; // Item value
+
+    public Item(int w, int v) {
+        this.w = w;
+        this.v = v;
+    }
+}
+
+public class fractional_knapsack {
+    /* Fractional knapsack: Greedy */
+    static double fractionalKnapsack(int[] wgt, int[] val, int cap) {
+        // Create an item list, containing two properties: weight, value
+        Item[] items = new Item[wgt.length];
+        for (int i = 0; i < wgt.length; i++) {
+            items[i] = new Item(wgt[i], val[i]);
+        }
+        // Sort by unit value item.v / item.w from high to low
+        Arrays.sort(items, Comparator.comparingDouble(item -> -((double) item.v / item.w)));
+        // Loop for greedy selection
+        double res = 0;
+        for (Item item : items) {
+            if (item.w <= cap) {
+                // If the remaining capacity is sufficient, put the entire item into the knapsack
+                res += item.v;
+                cap -= item.w;
+            } else {
+                // If the remaining capacity is insufficient, put part of the item into the knapsack
+                res += (double) item.v / item.w * cap;
+                // No remaining capacity left, thus break the loop
+                break;
+            }
+        }
+        return res;
+    }
+
+    public static void main(String[] args) {
+        int[] wgt = { 10, 20, 30, 40, 50 };
+        int[] val = { 50, 120, 150, 210, 240 };
+        int cap = 50;
+
+        // Greedy algorithm
+        double res = fractionalKnapsack(wgt, val, cap);
+        System.out.println("The maximum value within the bag capacity is " + res);
+    }
+}

en/codes/java/chapter_greedy/max_capacity.java

@@ -0,0 +1,38 @@
+/**
+ * File: max_capacity.java
+ * Created Time: 2023-07-21
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_greedy;
+
+public class max_capacity {
+    /* Maximum capacity: Greedy */
+    static int maxCapacity(int[] ht) {
+        // Initialize i, j, making them split the array at both ends
+        int i = 0, j = ht.length - 1;
+        // Initial maximum capacity is 0
+        int res = 0;
+        // Loop for greedy selection until the two boards meet
+        while (i < j) {
+            // Update maximum capacity
+            int cap = Math.min(ht[i], ht[j]) * (j - i);
+            res = Math.max(res, cap);
+            // Move the shorter board inward
+            if (ht[i] < ht[j]) {
+                i++;
+            } else {
+                j--;
+            }
+        }
+        return res;
+    }
+
+    public static void main(String[] args) {
+        int[] ht = { 3, 8, 5, 2, 7, 7, 3, 4 };
+
+        // Greedy algorithm
+        int res = maxCapacity(ht);
+        System.out.println("The maximum capacity is " + res);
+    }
+}

en/codes/java/chapter_greedy/max_product_cutting.java

@@ -0,0 +1,40 @@
+/**
+ * File: max_product_cutting.java
+ * Created Time: 2023-07-21
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_greedy;
+
+import java.lang.Math;
+
+public class max_product_cutting {
+    /* Maximum product of cutting: Greedy */
+    public static int maxProductCutting(int n) {
+        // When n <= 3, must cut out a 1
+        if (n <= 3) {
+            return 1 * (n - 1);
+        }
+        // Greedy cut out 3s, a is the number of 3s, b is the remainder
+        int a = n / 3;
+        int b = n % 3;
+        if (b == 1) {
+            // When the remainder is 1, convert a pair of 1 * 3 into 2 * 2
+            return (int) Math.pow(3, a - 1) * 2 * 2;
+        }
+        if (b == 2) {
+            // When the remainder is 2, do nothing
+            return (int) Math.pow(3, a) * 2;
+        }
+        // When the remainder is 0, do nothing
+        return (int) Math.pow(3, a);
+    }
+
+    public static void main(String[] args) {
+        int n = 58;
+
+        // Greedy algorithm
+        int res = maxProductCutting(n);
+        System.out.println("The maximum product of division is " + res);
+    }
+}

en/codes/java/chapter_hashing/array_hash_map.java

@@ -0,0 +1,141 @@
+/**
+ * File: array_hash_map.java
+ * Created Time: 2022-12-04
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_hashing;
+
+import java.util.*;
+
+/* Key-value pair */
+class Pair {
+    public int key;
+    public String val;
+
+    public Pair(int key, String val) {
+        this.key = key;
+        this.val = val;
+    }
+}
+
+/* Hash table based on array implementation */
+class ArrayHashMap {
+    private List<Pair> buckets;
+
+    public ArrayHashMap() {
+        // Initialize an array, containing 100 buckets
+        buckets = new ArrayList<>();
+        for (int i = 0; i < 100; i++) {
+            buckets.add(null);
+        }
+    }
+
+    /* Hash function */
+    private int hashFunc(int key) {
+        int index = key % 100;
+        return index;
+    }
+
+    /* Query operation */
+    public String get(int key) {
+        int index = hashFunc(key);
+        Pair pair = buckets.get(index);
+        if (pair == null)
+            return null;
+        return pair.val;
+    }
+
+    /* Add operation */
+    public void put(int key, String val) {
+        Pair pair = new Pair(key, val);
+        int index = hashFunc(key);
+        buckets.set(index, pair);
+    }
+
+    /* Remove operation */
+    public void remove(int key) {
+        int index = hashFunc(key);
+        // Set to null, indicating removal
+        buckets.set(index, null);
+    }
+
+    /* Get all key-value pairs */
+    public List<Pair> pairSet() {
+        List<Pair> pairSet = new ArrayList<>();
+        for (Pair pair : buckets) {
+            if (pair != null)
+                pairSet.add(pair);
+        }
+        return pairSet;
+    }
+
+    /* Get all keys */
+    public List<Integer> keySet() {
+        List<Integer> keySet = new ArrayList<>();
+        for (Pair pair : buckets) {
+            if (pair != null)
+                keySet.add(pair.key);
+        }
+        return keySet;
+    }
+
+    /* Get all values */
+    public List<String> valueSet() {
+        List<String> valueSet = new ArrayList<>();
+        for (Pair pair : buckets) {
+            if (pair != null)
+                valueSet.add(pair.val);
+        }
+        return valueSet;
+    }
+
+    /* Print hash table */
+    public void print() {
+        for (Pair kv : pairSet()) {
+            System.out.println(kv.key + " -> " + kv.val);
+        }
+    }
+}
+
+public class array_hash_map {
+    public static void main(String[] args) {
+        /* Initialize hash table */
+        ArrayHashMap map = new ArrayHashMap();
+
+        /* Add operation */
+        // Add key-value pair (key, value) to the hash table
+        map.put(12836, "Ha");
+        map.put(15937, "Luo");
+        map.put(16750, "Suan");
+        map.put(13276, "Fa");
+        map.put(10583, "Ya");
+        System.out.println("\nAfter adding, the hash table is\nKey -> Value");
+        map.print();
+
+        /* Query operation */
+        // Enter key to the hash table, get value
+        String name = map.get(15937);
+        System.out.println("\nEnter student ID 15937, found name " + name);
+
+        /* Remove operation */
+        // Remove key-value pair (key, value) from the hash table
+        map.remove(10583);
+        System.out.println("\nAfter removing 10583, the hash table is\nKey -> Value");
+        map.print();
+
+        /* Traverse hash table */
+        System.out.println("\nTraverse key-value pairs Key->Value");
+        for (Pair kv : map.pairSet()) {
+            System.out.println(kv.key + " -> " + kv.val);
+        }
+        System.out.println("\nIndividually traverse keys Key");
+        for (int key : map.keySet()) {
+            System.out.println(key);
+        }
+        System.out.println("\nIndividually traverse values Value");
+        for (String val : map.valueSet()) {
+            System.out.println(val);
+        }
+    }
+}

en/codes/java/chapter_hashing/built_in_hash.java

@@ -0,0 +1,38 @@
+/**
+ * File: built_in_hash.java
+ * Created Time: 2023-06-21
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_hashing;
+
+import utils.*;
+import java.util.*;
+
+public class built_in_hash {
+    public static void main(String[] args) {
+        int num = 3;
+        int hashNum = Integer.hashCode(num);
+        System.out.println("The hash value of integer " + num + " is " + hashNum);
+
+        boolean bol = true;
+        int hashBol = Boolean.hashCode(bol);
+        System.out.println("The hash value of boolean " + bol + " is " + hashBol);
+
+        double dec = 3.14159;
+        int hashDec = Double.hashCode(dec);
+        System.out.println("The hash value of decimal " + dec + " is " + hashDec);
+
+        String str = "Hello algorithm";
+        int hashStr = str.hashCode();
+        System.out.println("The hash value of string " + str + " is " + hashStr);
+
+        Object[] arr = { 12836, "Ha" };
+        int hashTup = Arrays.hashCode(arr);
+        System.out.println("The hash value of array " + Arrays.toString(arr) + " is " + hashTup);
+
+        ListNode obj = new ListNode(0);
+        int hashObj = obj.hashCode();
+        System.out.println("The hash value of node object " + obj + " is " + hashObj);
+    }
+}

en/codes/java/chapter_hashing/hash_map.java

@@ -0,0 +1,52 @@
+/**
+ * File: hash_map.java
+ * Created Time: 2022-12-04
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_hashing;
+
+import java.util.*;
+import utils.*;
+
+public class hash_map {
+    public static void main(String[] args) {
+        /* Initialize hash table */
+        Map<Integer, String> map = new HashMap<>();
+
+        /* Add operation */
+        // Add key-value pair (key, value) to the hash table
+        map.put(12836, "Ha");
+        map.put(15937, "Luo");
+        map.put(16750, "Suan");
+        map.put(13276, "Fa");
+        map.put(10583, "Ya");
+        System.out.println("\nAfter adding, the hash table is\nKey -> Value");
+        PrintUtil.printHashMap(map);
+
+        /* Query operation */
+        // Enter key to the hash table, get value
+        String name = map.get(15937);
+        System.out.println("\nEnter student ID 15937, found name " + name);
+
+        /* Remove operation */
+        // Remove key-value pair (key, value) from the hash table
+        map.remove(10583);
+        System.out.println("\nAfter removing 10583, the hash table is\nKey -> Value");
+        PrintUtil.printHashMap(map);
+
+        /* Traverse hash table */
+        System.out.println("\nTraverse key-value pairs Key->Value");
+        for (Map.Entry<Integer, String> kv : map.entrySet()) {
+            System.out.println(kv.getKey() + " -> " + kv.getValue());
+        }
+        System.out.println("\nIndividually traverse keys Key");
+        for (int key : map.keySet()) {
+            System.out.println(key);
+        }
+        System.out.println("\nIndividually traverse values Value");
+        for (String val : map.values()) {
+            System.out.println(val);
+        }
+    }
+}

en/codes/java/chapter_hashing/hash_map_chaining.java

@@ -0,0 +1,148 @@
+/**
+ * File: hash_map_chaining.java
+ * Created Time: 2023-06-13
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_hashing;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/* Chained address hash table */
+class HashMapChaining {
+    int size; // Number of key-value pairs
+    int capacity; // Hash table capacity
+    double loadThres; // Load factor threshold for triggering expansion
+    int extendRatio; // Expansion multiplier
+    List<List<Pair>> buckets; // Bucket array
+
+    /* Constructor */
+    public HashMapChaining() {
+        size = 0;
+        capacity = 4;
+        loadThres = 2.0 / 3.0;
+        extendRatio = 2;
+        buckets = new ArrayList<>(capacity);
+        for (int i = 0; i < capacity; i++) {
+            buckets.add(new ArrayList<>());
+        }
+    }
+
+    /* Hash function */
+    int hashFunc(int key) {
+        return key % capacity;
+    }
+
+    /* Load factor */
+    double loadFactor() {
+        return (double) size / capacity;
+    }
+
+    /* Query operation */
+    String get(int key) {
+        int index = hashFunc(key);
+        List<Pair> bucket = buckets.get(index);
+        // Traverse the bucket, if the key is found, return the corresponding val
+        for (Pair pair : bucket) {
+            if (pair.key == key) {
+                return pair.val;
+            }
+        }
+        // If key is not found, return null
+        return null;
+    }
+
+    /* Add operation */
+    void put(int key, String val) {
+        // When the load factor exceeds the threshold, perform expansion
+        if (loadFactor() > loadThres) {
+            extend();
+        }
+        int index = hashFunc(key);
+        List<Pair> bucket = buckets.get(index);
+        // Traverse the bucket, if the specified key is encountered, update the corresponding val and return
+        for (Pair pair : bucket) {
+            if (pair.key == key) {
+                pair.val = val;
+                return;
+            }
+        }
+        // If the key is not found, add the key-value pair to the end
+        Pair pair = new Pair(key, val);
+        bucket.add(pair);
+        size++;
+    }
+
+    /* Remove operation */
+    void remove(int key) {
+        int index = hashFunc(key);
+        List<Pair> bucket = buckets.get(index);
+        // Traverse the bucket, remove the key-value pair from it
+        for (Pair pair : bucket) {
+            if (pair.key == key) {
+                bucket.remove(pair);
+                size--;
+                break;
+            }
+        }
+    }
+
+    /* Extend hash table */
+    void extend() {
+        // Temporarily store the original hash table
+        List<List<Pair>> bucketsTmp = buckets;
+        // Initialize the extended new hash table
+        capacity *= extendRatio;
+        buckets = new ArrayList<>(capacity);
+        for (int i = 0; i < capacity; i++) {
+            buckets.add(new ArrayList<>());
+        }
+        size = 0;
+        // Move key-value pairs from the original hash table to the new hash table
+        for (List<Pair> bucket : bucketsTmp) {
+            for (Pair pair : bucket) {
+                put(pair.key, pair.val);
+            }
+        }
+    }
+
+    /* Print hash table */
+    void print() {
+        for (List<Pair> bucket : buckets) {
+            List<String> res = new ArrayList<>();
+            for (Pair pair : bucket) {
+                res.add(pair.key + " -> " + pair.val);
+            }
+            System.out.println(res);
+        }
+    }
+}
+
+public class hash_map_chaining {
+    public static void main(String[] args) {
+        /* Initialize hash table */
+        HashMapChaining map = new HashMapChaining();
+
+        /* Add operation */
+        // Add key-value pair (key, value) to the hash table
+        map.put(12836, "Ha");
+        map.put(15937, "Luo");
+        map.put(16750, "Suan");
+        map.put(13276, "Fa");
+        map.put(10583, "Ya");
+        System.out.println("\nAfter adding, the hash table is\nKey -> Value");
+        map.print();
+
+        /* Query operation */
+        // Enter key to the hash table, get value
+        String name = map.get(13276);
+        System.out.println("\nEnter student ID 13276, found name " + name);
+
+        /* Remove operation */
+        // Remove key-value pair (key, value) from the hash table
+        map.remove(12836);
+        System.out.println("\nAfter removing 12836, the hash table is\nKey -> Value");
+        map.print();
+    }
+}

en/codes/java/chapter_hashing/hash_map_open_addressing.java

@@ -0,0 +1,158 @@
+/**
+ * File: hash_map_open_addressing.java
+ * Created Time: 2023-06-13
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_hashing;
+
+/* Open addressing hash table */
+class HashMapOpenAddressing {
+    private int size; // Number of key-value pairs
+    private int capacity = 4; // Hash table capacity
+    private final double loadThres = 2.0 / 3.0; // Load factor threshold for triggering expansion
+    private final int extendRatio = 2; // Expansion multiplier
+    private Pair[] buckets; // Bucket array
+    private final Pair TOMBSTONE = new Pair(-1, "-1"); // Removal mark
+
+    /* Constructor */
+    public HashMapOpenAddressing() {
+        size = 0;
+        buckets = new Pair[capacity];
+    }
+
+    /* Hash function */
+    private int hashFunc(int key) {
+        return key % capacity;
+    }
+
+    /* Load factor */
+    private double loadFactor() {
+        return (double) size / capacity;
+    }
+
+    /* Search for the bucket index corresponding to key */
+    private int findBucket(int key) {
+        int index = hashFunc(key);
+        int firstTombstone = -1;
+        // Linear probing, break when encountering an empty bucket
+        while (buckets[index] != null) {
+            // If the key is encountered, return the corresponding bucket index
+            if (buckets[index].key == key) {
+                // If a removal mark was encountered earlier, move the key-value pair to that index
+                if (firstTombstone != -1) {
+                    buckets[firstTombstone] = buckets[index];
+                    buckets[index] = TOMBSTONE;
+                    return firstTombstone; // Return the moved bucket index
+                }
+                return index; // Return bucket index
+            }
+            // Record the first encountered removal mark
+            if (firstTombstone == -1 && buckets[index] == TOMBSTONE) {
+                firstTombstone = index;
+            }
+            // Calculate the bucket index, return to the head if exceeding the tail
+            index = (index + 1) % capacity;
+        }
+        // If the key does not exist, return the index of the insertion point
+        return firstTombstone == -1 ? index : firstTombstone;
+    }
+
+    /* Query operation */
+    public String get(int key) {
+        // Search for the bucket index corresponding to key
+        int index = findBucket(key);
+        // If the key-value pair is found, return the corresponding val
+        if (buckets[index] != null && buckets[index] != TOMBSTONE) {
+            return buckets[index].val;
+        }
+        // If the key-value pair does not exist, return null
+        return null;
+    }
+
+    /* Add operation */
+    public void put(int key, String val) {
+        // When the load factor exceeds the threshold, perform expansion
+        if (loadFactor() > loadThres) {
+            extend();
+        }
+        // Search for the bucket index corresponding to key
+        int index = findBucket(key);
+        // If the key-value pair is found, overwrite val and return
+        if (buckets[index] != null && buckets[index] != TOMBSTONE) {
+            buckets[index].val = val;
+            return;
+        }
+        // If the key-value pair does not exist, add the key-value pair
+        buckets[index] = new Pair(key, val);
+        size++;
+    }
+
+    /* Remove operation */
+    public void remove(int key) {
+        // Search for the bucket index corresponding to key
+        int index = findBucket(key);
+        // If the key-value pair is found, cover it with a removal mark
+        if (buckets[index] != null && buckets[index] != TOMBSTONE) {
+            buckets[index] = TOMBSTONE;
+            size--;
+        }
+    }
+
+    /* Extend hash table */
+    private void extend() {
+        // Temporarily store the original hash table
+        Pair[] bucketsTmp = buckets;
+        // Initialize the extended new hash table
+        capacity *= extendRatio;
+        buckets = new Pair[capacity];
+        size = 0;
+        // Move key-value pairs from the original hash table to the new hash table
+        for (Pair pair : bucketsTmp) {
+            if (pair != null && pair != TOMBSTONE) {
+                put(pair.key, pair.val);
+            }
+        }
+    }
+
+    /* Print hash table */
+    public void print() {
+        for (Pair pair : buckets) {
+            if (pair == null) {
+                System.out.println("null");
+            } else if (pair == TOMBSTONE) {
+                System.out.println("TOMBSTONE");
+            } else {
+                System.out.println(pair.key + " -> " + pair.val);
+            }
+        }
+    }
+}
+
+public class hash_map_open_addressing {
+    public static void main(String[] args) {
+        // Initialize hash table
+        HashMapOpenAddressing hashmap = new HashMapOpenAddressing();
+
+        // Add operation
+        // Add key-value pair (key, val) to the hash table
+        hashmap.put(12836, "Ha");
+        hashmap.put(15937, "Luo");
+        hashmap.put(16750, "Suan");
+        hashmap.put(13276, "Fa");
+        hashmap.put(10583, "Ya");
+        System.out.println("\nAfter adding, the hash table is\nKey -> Value");
+        hashmap.print();
+
+        // Query operation
+        // Enter key to the hash table, get value val
+        String name = hashmap.get(13276);
+        System.out.println("\nEnter student ID 13276, found name " + name);
+
+        // Remove operation
+        // Remove key-value pair (key, val) from the hash table
+        hashmap.remove(16750);
+        System.out.println("\nAfter removing 16750, the hash table is\nKey -> Value");
+        hashmap.print();
+    }
+}

en/codes/java/chapter_hashing/simple_hash.java

@@ -0,0 +1,65 @@
+/**
+ * File: simple_hash.java
+ * Created Time: 2023-06-21
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_hashing;
+
+public class simple_hash {
+    /* Additive hash */
+    static int addHash(String key) {
+        long hash = 0;
+        final int MODULUS = 1000000007;
+        for (char c : key.toCharArray()) {
+            hash = (hash + (int) c) % MODULUS;
+        }
+        return (int) hash;
+    }
+
+    /* Multiplicative hash */
+    static int mulHash(String key) {
+        long hash = 0;
+        final int MODULUS = 1000000007;
+        for (char c : key.toCharArray()) {
+            hash = (31 * hash + (int) c) % MODULUS;
+        }
+        return (int) hash;
+    }
+
+    /* XOR hash */
+    static int xorHash(String key) {
+        int hash = 0;
+        final int MODULUS = 1000000007;
+        for (char c : key.toCharArray()) {
+            hash ^= (int) c;
+        }
+        return hash & MODULUS;
+    }
+
+    /* Rotational hash */
+    static int rotHash(String key) {
+        long hash = 0;
+        final int MODULUS = 1000000007;
+        for (char c : key.toCharArray()) {
+            hash = ((hash << 4) ^ (hash >> 28) ^ (int) c) % MODULUS;
+        }
+        return (int) hash;
+    }
+
+    public static void main(String[] args) {
+        String key = "Hello algorithm";
+
+        int hash = addHash(key);
+        System.out.println("Additive hash value is " + hash);
+
+        hash = mulHash(key);
+        System.out.println("Multiplicative hash value is " + hash);
+
+        hash = xorHash(key);
+        System.out.println("XOR hash value is " + hash);
+
+        hash = rotHash(key);
+        System.out.println("Rotational hash value is " + hash);
+    }
+}

en/codes/java/chapter_heap/heap.java

@@ -0,0 +1,66 @@
+/**
+ * File: heap.java
+ * Created Time: 2023-01-07
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_heap;
+
+import utils.*;
+import java.util.*;
+
+public class heap {
+    public static void testPush(Queue<Integer> heap, int val) {
+        heap.offer(val); // Push the element into heap
+        System.out.format("\nAfter element %d is added to the heap\n", val);
+        PrintUtil.printHeap(heap);
+    }
+
+    public static void testPop(Queue<Integer> heap) {
+        int val = heap.poll(); // Pop the element at the heap top
+        System.out.format("\nAfter the top element %d is removed from the heap\n", val);
+        PrintUtil.printHeap(heap);
+    }
+
+    public static void main(String[] args) {
+        /* Initialize the heap */
+        // Initialize min-heap
+        Queue<Integer> minHeap = new PriorityQueue<>();
+        // Initialize the max-heap (using lambda expression to modify Comparator if necessary)
+        Queue<Integer> maxHeap = new PriorityQueue<>((a, b) -> b - a);
+
+        System.out.println("\nThe following test case is for max-heap");
+
+        /* Push the element into heap */
+        testPush(maxHeap, 1);
+        testPush(maxHeap, 3);
+        testPush(maxHeap, 2);
+        testPush(maxHeap, 5);
+        testPush(maxHeap, 4);
+
+        /* Access heap top element */
+        int peek = maxHeap.peek();
+        System.out.format("\nTop element of the heap is %d\n", peek);
+
+        /* Pop the element at the heap top */
+        testPop(maxHeap);
+        testPop(maxHeap);
+        testPop(maxHeap);
+        testPop(maxHeap);
+        testPop(maxHeap);
+
+        /* Get heap size */
+        int size = maxHeap.size();
+        System.out.format("\nNumber of elements in the heap is %d\n", size);
+
+        /* Determine if heap is empty */
+        boolean isEmpty = maxHeap.isEmpty();
+        System.out.format("\nIs the heap empty %b\n", isEmpty);
+
+        /* Enter list and build heap */
+        // Time complexity is O(n), not O(nlogn)
+        minHeap = new PriorityQueue<>(Arrays.asList(1, 3, 2, 5, 4));
+        System.out.println("\nEnter list and build min-heap");
+        PrintUtil.printHeap(minHeap);
+    }
+}

en/codes/java/chapter_heap/my_heap.java

@@ -0,0 +1,159 @@
+/**
+ * File: my_heap.java
+ * Created Time: 2023-01-07
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_heap;
+
+import utils.*;
+import java.util.*;
+
+/* Max-heap */
+class MaxHeap {
+    // Use list instead of array to avoid the need for resizing
+    private List<Integer> maxHeap;
+
+    /* Constructor, build heap based on input list */
+    public MaxHeap(List<Integer> nums) {
+        // Add all list elements into the heap
+        maxHeap = new ArrayList<>(nums);
+        // Heapify all nodes except leaves
+        for (int i = parent(size() - 1); i >= 0; i--) {
+            siftDown(i);
+        }
+    }
+
+    /* Get index of left child node */
+    private int left(int i) {
+        return 2 * i + 1;
+    }
+
+    /* Get index of right child node */
+    private int right(int i) {
+        return 2 * i + 2;
+    }
+
+    /* Get index of parent node */
+    private int parent(int i) {
+        return (i - 1) / 2; // Integer division down
+    }
+
+    /* Swap elements */
+    private void swap(int i, int j) {
+        int tmp = maxHeap.get(i);
+        maxHeap.set(i, maxHeap.get(j));
+        maxHeap.set(j, tmp);
+    }
+
+    /* Get heap size */
+    public int size() {
+        return maxHeap.size();
+    }
+
+    /* Determine if heap is empty */
+    public boolean isEmpty() {
+        return size() == 0;
+    }
+
+    /* Access heap top element */
+    public int peek() {
+        return maxHeap.get(0);
+    }
+
+    /* Push the element into heap */
+    public void push(int val) {
+        // Add node
+        maxHeap.add(val);
+        // Heapify from bottom to top
+        siftUp(size() - 1);
+    }
+
+    /* Start heapifying node i, from bottom to top */
+    private void siftUp(int i) {
+        while (true) {
+            // Get parent node of node i
+            int p = parent(i);
+            // When "crossing the root node" or "node does not need repair", end heapification
+            if (p < 0 || maxHeap.get(i) <= maxHeap.get(p))
+                break;
+            // Swap two nodes
+            swap(i, p);
+            // Loop upwards heapification
+            i = p;
+        }
+    }
+
+    /* Element exits heap */
+    public int pop() {
+        // Empty handling
+        if (isEmpty())
+            throw new IndexOutOfBoundsException();
+        // Swap the root node with the rightmost leaf node (swap the first element with the last element)
+        swap(0, size() - 1);
+        // Remove node
+        int val = maxHeap.remove(size() - 1);
+        // Heapify from top to bottom
+        siftDown(0);
+        // Return heap top element
+        return val;
+    }
+
+    /* Start heapifying node i, from top to bottom */
+    private void siftDown(int i) {
+        while (true) {
+            // Determine the largest node among i, l, r, noted as ma
+            int l = left(i), r = right(i), ma = i;
+            if (l < size() && maxHeap.get(l) > maxHeap.get(ma))
+                ma = l;
+            if (r < size() && maxHeap.get(r) > maxHeap.get(ma))
+                ma = r;
+            // If node i is the largest or indices l, r are out of bounds, no further heapification needed, break
+            if (ma == i)
+                break;
+            // Swap two nodes
+            swap(i, ma);
+            // Loop downwards heapification
+            i = ma;
+        }
+    }
+
+    /* Print heap (binary tree) */
+    public void print() {
+        Queue<Integer> queue = new PriorityQueue<>((a, b) -> { return b - a; });
+        queue.addAll(maxHeap);
+        PrintUtil.printHeap(queue);
+    }
+}
+
+public class my_heap {
+    public static void main(String[] args) {
+        /* Initialize max-heap */
+        MaxHeap maxHeap = new MaxHeap(Arrays.asList(9, 8, 6, 6, 7, 5, 2, 1, 4, 3, 6, 2));
+        System.out.println("\nEnter list and build heap");
+        maxHeap.print();
+
+        /* Access heap top element */
+        int peek = maxHeap.peek();
+        System.out.format("\nTop element of the heap is %d\n", peek);
+
+        /* Push the element into heap */
+        int val = 7;
+        maxHeap.push(val);
+        System.out.format("\nAfter element %d is added to the heap\n", val);
+        maxHeap.print();
+
+        /* Pop the element at the heap top */
+        peek = maxHeap.pop();
+        System.out.format("\nAfter the top element %d is removed from the heap\n", peek);
+        maxHeap.print();
+
+        /* Get heap size */
+        int size = maxHeap.size();
+        System.out.format("\nNumber of elements in the heap is %d\n", size);
+
+        /* Determine if heap is empty */
+        boolean isEmpty = maxHeap.isEmpty();
+        System.out.format("\nIs the heap empty %b\n", isEmpty);
+    }
+}

en/codes/java/chapter_heap/top_k.java

@@ -0,0 +1,40 @@
+/**
+ * File: top_k.java
+ * Created Time: 2023-06-12
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_heap;
+
+import utils.*;
+import java.util.*;
+
+public class top_k {
+    /* Using heap to find the largest k elements in an array */
+    static Queue<Integer> topKHeap(int[] nums, int k) {
+        // Initialize min-heap
+        Queue<Integer> heap = new PriorityQueue<Integer>();
+        // Enter the first k elements of the array into the heap
+        for (int i = 0; i < k; i++) {
+            heap.offer(nums[i]);
+        }
+        // From the k+1th element, keep the heap length as k
+        for (int i = k; i < nums.length; i++) {
+            // If the current element is larger than the heap top element, remove the heap top element and enter the current element into the heap
+            if (nums[i] > heap.peek()) {
+                heap.poll();
+                heap.offer(nums[i]);
+            }
+        }
+        return heap;
+    }
+
+    public static void main(String[] args) {
+        int[] nums = { 1, 7, 6, 3, 2 };
+        int k = 3;
+
+        Queue<Integer> res = topKHeap(nums, k);
+        System.out.println("The largest " + k + " elements are");
+        PrintUtil.printHeap(res);
+    }
+}

en/codes/java/chapter_searching/binary_search.java

@@ -0,0 +1,58 @@
+/**
+ * File: binary_search.java
+ * Created Time: 2022-11-25
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_searching;
+
+public class binary_search {
+    /* Binary search (double closed interval) */
+    static int binarySearch(int[] nums, int target) {
+        // Initialize double closed interval [0, n-1], i.e., i, j point to the first element and last element of the array respectively
+        int i = 0, j = nums.length - 1;
+        // Loop until the search interval is empty (when i > j, it is empty)
+        while (i <= j) {
+            int m = i + (j - i) / 2; // Calculate midpoint index m
+            if (nums[m] < target) // This situation indicates that target is in the interval [m+1, j]
+                i = m + 1;
+            else if (nums[m] > target) // This situation indicates that target is in the interval [i, m-1]
+                j = m - 1;
+            else // Found the target element, thus return its index
+                return m;
+        }
+        // Did not find the target element, thus return -1
+        return -1;
+    }
+
+    /* Binary search (left closed right open interval) */
+    static int binarySearchLCRO(int[] nums, int target) {
+        // Initialize left closed right open interval [0, n), i.e., i, j point to the first element and the last element +1 of the array respectively
+        int i = 0, j = nums.length;
+        // Loop until the search interval is empty (when i = j, it is empty)
+        while (i < j) {
+            int m = i + (j - i) / 2; // Calculate midpoint index m
+            if (nums[m] < target) // This situation indicates that target is in the interval [m+1, j)
+                i = m + 1;
+            else if (nums[m] > target) // This situation indicates that target is in the interval [i, m)
+                j = m;
+            else // Found the target element, thus return its index
+                return m;
+        }
+        // Did not find the target element, thus return -1
+        return -1;
+    }
+
+    public static void main(String[] args) {
+        int target = 6;
+        int[] nums = { 1, 3, 6, 8, 12, 15, 23, 26, 31, 35 };
+
+        /* Binary search (double closed interval) */
+        int index = binarySearch(nums, target);
+        System.out.println("Index of target element 6 =" + index);
+
+        /* Binary search (left closed right open interval) */
+        index = binarySearchLCRO(nums, target);
+        System.out.println("Index of target element 6 =" + index);
+    }
+}

en/codes/java/chapter_searching/binary_search_edge.java

@@ -0,0 +1,49 @@
+/**
+ * File: binary_search_edge.java
+ * Created Time: 2023-08-04
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_searching;
+
+public class binary_search_edge {
+    /* Binary search for the leftmost target */
+    static int binarySearchLeftEdge(int[] nums, int target) {
+        // Equivalent to finding the insertion point of target
+        int i = binary_search_insertion.binarySearchInsertion(nums, target);
+        // Did not find target, thus return -1
+        if (i == nums.length || nums[i] != target) {
+            return -1;
+        }
+        // Found target, return index i
+        return i;
+    }
+
+    /* Binary search for the rightmost target */
+    static int binarySearchRightEdge(int[] nums, int target) {
+        // Convert to finding the leftmost target + 1
+        int i = binary_search_insertion.binarySearchInsertion(nums, target + 1);
+        // j points to the rightmost target, i points to the first element greater than target
+        int j = i - 1;
+        // Did not find target, thus return -1
+        if (j == -1 || nums[j] != target) {
+            return -1;
+        }
+        // Found target, return index j
+        return j;
+    }
+
+    public static void main(String[] args) {
+        // Array with duplicate elements
+        int[] nums = { 1, 3, 6, 6, 6, 6, 6, 10, 12, 15 };
+        System.out.println("\nArray nums = " + java.util.Arrays.toString(nums));
+
+        // Binary search for left and right boundaries
+        for (int target : new int[] { 6, 7 }) {
+            int index = binarySearchLeftEdge(nums, target);
+            System.out.println("The leftmost index of element " + target + " is " + index);
+            index = binarySearchRightEdge(nums, target);
+            System.out.println("The rightmost index of element " + target + " is " + index);
+        }
+    }
+}

en/codes/java/chapter_searching/binary_search_insertion.java

@@ -0,0 +1,63 @@
+/**
+ * File: binary_search_insertion.java
+ * Created Time: 2023-08-04
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_searching;
+
+class binary_search_insertion {
+    /* Binary search for insertion point (no duplicate elements) */
+    static int binarySearchInsertionSimple(int[] nums, int target) {
+        int i = 0, j = nums.length - 1; // Initialize double closed interval [0, n-1]
+        while (i <= j) {
+            int m = i + (j - i) / 2; // Calculate midpoint index m
+            if (nums[m] < target) {
+                i = m + 1; // Target is in interval [m+1, j]
+            } else if (nums[m] > target) {
+                j = m - 1; // Target is in interval [i, m-1]
+            } else {
+                return m; // Found target, return insertion point m
+            }
+        }
+        // Did not find target, return insertion point i
+        return i;
+    }
+
+    /* Binary search for insertion point (with duplicate elements) */
+    static int binarySearchInsertion(int[] nums, int target) {
+        int i = 0, j = nums.length - 1; // Initialize double closed interval [0, n-1]
+        while (i <= j) {
+            int m = i + (j - i) / 2; // Calculate midpoint index m
+            if (nums[m] < target) {
+                i = m + 1; // Target is in interval [m+1, j]
+            } else if (nums[m] > target) {
+                j = m - 1; // Target is in interval [i, m-1]
+            } else {
+                j = m - 1; // First element less than target is in interval [i, m-1]
+            }
+        }
+        // Return insertion point i
+        return i;
+    }
+
+    public static void main(String[] args) {
+        // Array without duplicate elements
+        int[] nums = { 1, 3, 6, 8, 12, 15, 23, 26, 31, 35 };
+        System.out.println("\nArray nums = " + java.util.Arrays.toString(nums));
+        // Binary search for insertion point
+        for (int target : new int[] { 6, 9 }) {
+            int index = binarySearchInsertionSimple(nums, target);
+            System.out.println("The insertion point index for element " + target + " is " + index);
+        }
+
+        // Array with duplicate elements
+        nums = new int[] { 1, 3, 6, 6, 6, 6, 6, 10, 12, 15 };
+        System.out.println("\nArray nums = " + java.util.Arrays.toString(nums));
+        // Binary search for insertion point
+        for (int target : new int[] { 2, 6, 20 }) {
+            int index = binarySearchInsertion(nums, target);
+            System.out.println("The insertion point index for element " + target + " is " + index);
+        }
+    }
+}

en/codes/java/chapter_searching/hashing_search.java

@@ -0,0 +1,51 @@
+/**
+ * File: hashing_search.java
+ * Created Time: 2022-11-25
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_searching;
+
+import utils.*;
+import java.util.*;
+
+public class hashing_search {
+    /* Hash search (array) */
+    static int hashingSearchArray(Map<Integer, Integer> map, int target) {
+        // Hash table's key: target element, value: index
+        // If the hash table does not contain this key, return -1
+        return map.getOrDefault(target, -1);
+    }
+
+    /* Hash search (linked list) */
+    static ListNode hashingSearchLinkedList(Map<Integer, ListNode> map, int target) {
+        // Hash table key: target node value, value: node object
+        // If the key is not in the hash table, return null
+        return map.getOrDefault(target, null);
+    }
+
+    public static void main(String[] args) {
+        int target = 3;
+
+        /* Hash search (array) */
+        int[] nums = { 1, 5, 3, 2, 4, 7, 5, 9, 10, 8 };
+        // Initialize hash table
+        Map<Integer, Integer> map = new HashMap<>();
+        for (int i = 0; i < nums.length; i++) {
+            map.put(nums[i], i); // key: element, value: index
+        }
+        int index = hashingSearchArray(map, target);
+        System.out.println("The index of target element 3 is " + index);
+
+        /* Hash search (linked list) */
+        ListNode head = ListNode.arrToLinkedList(nums);
+        // Initialize hash table
+        Map<Integer, ListNode> map1 = new HashMap<>();
+        while (head != null) {
+            map1.put(head.val, head); // key: node value, value: node
+            head = head.next;
+        }
+        ListNode node = hashingSearchLinkedList(map1, target);
+        System.out.println("The corresponding node object for target node value 3 is " + node);
+    }
+}

en/codes/java/chapter_searching/linear_search.java

@@ -0,0 +1,50 @@
+/**
+ * File: linear_search.java
+ * Created Time: 2022-11-25
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_searching;
+
+import utils.*;
+
+public class linear_search {
+    /* Linear search (array) */
+    static int linearSearchArray(int[] nums, int target) {
+        // Traverse array
+        for (int i = 0; i < nums.length; i++) {
+            // Found the target element, thus return its index
+            if (nums[i] == target)
+                return i;
+        }
+        // Did not find the target element, thus return -1
+        return -1;
+    }
+
+    /* Linear search (linked list) */
+    static ListNode linearSearchLinkedList(ListNode head, int target) {
+        // Traverse the list
+        while (head != null) {
+            // Found the target node, return it
+            if (head.val == target)
+                return head;
+            head = head.next;
+        }
+        // If the target node is not found, return null
+        return null;
+    }
+
+    public static void main(String[] args) {
+        int target = 3;
+
+        /* Perform linear search in array */
+        int[] nums = { 1, 5, 3, 2, 4, 7, 5, 9, 10, 8 };
+        int index = linearSearchArray(nums, target);
+        System.out.println("The index of target element 3 is " + index);
+
+        /* Perform linear search in linked list */
+        ListNode head = ListNode.arrToLinkedList(nums);
+        ListNode node = linearSearchLinkedList(head, target);
+        System.out.println("The corresponding node object for target node value 3 is " + node);
+    }
+}

en/codes/java/chapter_searching/two_sum.java

@@ -0,0 +1,53 @@
+/**
+ * File: two_sum.java
+ * Created Time: 2022-11-25
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_searching;
+
+import java.util.*;
+
+public class two_sum {
+    /* Method one: Brute force enumeration */
+    static int[] twoSumBruteForce(int[] nums, int target) {
+        int size = nums.length;
+        // Two-layer loop, time complexity is O(n^2)
+        for (int i = 0; i < size - 1; i++) {
+            for (int j = i + 1; j < size; j++) {
+                if (nums[i] + nums[j] == target)
+                    return new int[] { i, j };
+            }
+        }
+        return new int[0];
+    }
+
+    /* Method two: Auxiliary hash table */
+    static int[] twoSumHashTable(int[] nums, int target) {
+        int size = nums.length;
+        // Auxiliary hash table, space complexity is O(n)
+        Map<Integer, Integer> dic = new HashMap<>();
+        // Single-layer loop, time complexity is O(n)
+        for (int i = 0; i < size; i++) {
+            if (dic.containsKey(target - nums[i])) {
+                return new int[] { dic.get(target - nums[i]), i };
+            }
+            dic.put(nums[i], i);
+        }
+        return new int[0];
+    }
+
+    public static void main(String[] args) {
+        // ======= Test Case =======
+        int[] nums = { 2, 7, 11, 15 };
+        int target = 13;
+
+        // ====== Driver Code ======
+        // Method one
+        int[] res = twoSumBruteForce(nums, target);
+        System.out.println("Method one res = " + Arrays.toString(res));
+        // Method two
+        res = twoSumHashTable(nums, target);
+        System.out.println("Method two res = " + Arrays.toString(res));
+    }
+}

en/codes/java/chapter_sorting/bubble_sort.java

@@ -0,0 +1,57 @@
+/**
+ * File: bubble_sort.java
+ * Created Time: 2022-11-25
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_sorting;
+
+import java.util.*;
+
+public class bubble_sort {
+    /* Bubble sort */
+    static void bubbleSort(int[] nums) {
+        // Outer loop: unsorted range is [0, i]
+        for (int i = nums.length - 1; i > 0; i--) {
+            // Inner loop: swap the largest element in the unsorted range [0, i] to the right end of the range
+            for (int j = 0; j < i; j++) {
+                if (nums[j] > nums[j + 1]) {
+                    // Swap nums[j] and nums[j + 1]
+                    int tmp = nums[j];
+                    nums[j] = nums[j + 1];
+                    nums[j + 1] = tmp;
+                }
+            }
+        }
+    }
+
+    /* Bubble sort (optimized with flag) */
+    static void bubbleSortWithFlag(int[] nums) {
+        // Outer loop: unsorted range is [0, i]
+        for (int i = nums.length - 1; i > 0; i--) {
+            boolean flag = false; // Initialize flag
+            // Inner loop: swap the largest element in the unsorted range [0, i] to the right end of the range
+            for (int j = 0; j < i; j++) {
+                if (nums[j] > nums[j + 1]) {
+                    // Swap nums[j] and nums[j + 1]
+                    int tmp = nums[j];
+                    nums[j] = nums[j + 1];
+                    nums[j + 1] = tmp;
+                    flag = true; // Record swapped elements
+                }
+            }
+            if (!flag)
+                break; // If no elements were swapped in this round of "bubbling", exit
+        }
+    }
+
+    public static void main(String[] args) {
+        int[] nums = { 4, 1, 3, 1, 5, 2 };
+        bubbleSort(nums);
+        System.out.println("After bubble sort, nums = " + Arrays.toString(nums));
+
+        int[] nums1 = { 4, 1, 3, 1, 5, 2 };
+        bubbleSortWithFlag(nums1);
+        System.out.println("After bubble sort, nums1 = " + Arrays.toString(nums1));
+    }
+}

en/codes/java/chapter_sorting/bucket_sort.java

@@ -0,0 +1,47 @@
+/**
+ * File: bucket_sort.java
+ * Created Time: 2023-03-17
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_sorting;
+
+import java.util.*;
+
+public class bucket_sort {
+    /* Bucket sort */
+    static void bucketSort(float[] nums) {
+        // Initialize k = n/2 buckets, expected to allocate 2 elements per bucket
+        int k = nums.length / 2;
+        List<List<Float>> buckets = new ArrayList<>();
+        for (int i = 0; i < k; i++) {
+            buckets.add(new ArrayList<>());
+        }
+        // 1. Distribute array elements into various buckets
+        for (float num : nums) {
+            // Input data range is [0, 1), use num * k to map to index range [0, k-1]
+            int i = (int) (num * k);
+            // Add num to bucket i
+            buckets.get(i).add(num);
+        }
+        // 2. Sort each bucket
+        for (List<Float> bucket : buckets) {
+            // Use built-in sorting function, can also replace with other sorting algorithms
+            Collections.sort(bucket);
+        }
+        // 3. Traverse buckets to merge results
+        int i = 0;
+        for (List<Float> bucket : buckets) {
+            for (float num : bucket) {
+                nums[i++] = num;
+            }
+        }
+    }
+
+    public static void main(String[] args) {
+        // Assume input data is floating point, range [0, 1)
+        float[] nums = { 0.49f, 0.96f, 0.82f, 0.09f, 0.57f, 0.43f, 0.91f, 0.75f, 0.15f, 0.37f };
+        bucketSort(nums);
+        System.out.println("After bucket sort, nums = " + Arrays.toString(nums));
+    }
+}

en/codes/java/chapter_sorting/counting_sort.java

@@ -0,0 +1,78 @@
+/**
+ * File: counting_sort.java
+ * Created Time: 2023-03-17
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_sorting;
+
+import java.util.*;
+
+public class counting_sort {
+    /* Counting sort */
+    // Simple implementation, cannot be used for sorting objects
+    static void countingSortNaive(int[] nums) {
+        // 1. Count the maximum element m in the array
+        int m = 0;
+        for (int num : nums) {
+            m = Math.max(m, num);
+        }
+        // 2. Count the occurrence of each digit
+        // counter[num] represents the occurrence of num
+        int[] counter = new int[m + 1];
+        for (int num : nums) {
+            counter[num]++;
+        }
+        // 3. Traverse counter, filling each element back into the original array nums
+        int i = 0;
+        for (int num = 0; num < m + 1; num++) {
+            for (int j = 0; j < counter[num]; j++, i++) {
+                nums[i] = num;
+            }
+        }
+    }
+
+    /* Counting sort */
+    // Complete implementation, can sort objects and is a stable sort
+    static void countingSort(int[] nums) {
+        // 1. Count the maximum element m in the array
+        int m = 0;
+        for (int num : nums) {
+            m = Math.max(m, num);
+        }
+        // 2. Count the occurrence of each digit
+        // counter[num] represents the occurrence of num
+        int[] counter = new int[m + 1];
+        for (int num : nums) {
+            counter[num]++;
+        }
+        // 3. Calculate the prefix sum of counter, converting "occurrence count" to "tail index"
+        // counter[num]-1 is the last index where num appears in res
+        for (int i = 0; i < m; i++) {
+            counter[i + 1] += counter[i];
+        }
+        // 4. Traverse nums in reverse order, placing each element into the result array res
+        // Initialize the array res to record results
+        int n = nums.length;
+        int[] res = new int[n];
+        for (int i = n - 1; i >= 0; i--) {
+            int num = nums[i];
+            res[counter[num] - 1] = num; // Place num at the corresponding index
+            counter[num]--; // Decrement the prefix sum by 1, getting the next index to place num
+        }
+        // Use result array res to overwrite the original array nums
+        for (int i = 0; i < n; i++) {
+            nums[i] = res[i];
+        }
+    }
+
+    public static void main(String[] args) {
+        int[] nums = { 1, 0, 1, 2, 0, 4, 0, 2, 2, 4 };
+        countingSortNaive(nums);
+        System.out.println("After count sort (unable to sort objects), nums = " + Arrays.toString(nums));
+
+        int[] nums1 = { 1, 0, 1, 2, 0, 4, 0, 2, 2, 4 };
+        countingSort(nums1);
+        System.out.println("After count sort, nums1 = " + Arrays.toString(nums1));
+    }
+}

en/codes/java/chapter_sorting/heap_sort.java

@@ -0,0 +1,57 @@
+/**
+ * File: heap_sort.java
+ * Created Time: 2023-05-26
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_sorting;
+
+import java.util.Arrays;
+
+public class heap_sort {
+    /* Heap length is n, start heapifying node i, from top to bottom */
+    public static void siftDown(int[] nums, int n, int i) {
+        while (true) {
+            // Determine the largest node among i, l, r, noted as ma
+            int l = 2 * i + 1;
+            int r = 2 * i + 2;
+            int ma = i;
+            if (l < n && nums[l] > nums[ma])
+                ma = l;
+            if (r < n && nums[r] > nums[ma])
+                ma = r;
+            // If node i is the largest or indices l, r are out of bounds, no further heapification needed, break
+            if (ma == i)
+                break;
+            // Swap two nodes
+            int temp = nums[i];
+            nums[i] = nums[ma];
+            nums[ma] = temp;
+            // Loop downwards heapification
+            i = ma;
+        }
+    }
+
+    /* Heap sort */
+    public static void heapSort(int[] nums) {
+        // Build heap operation: heapify all nodes except leaves
+        for (int i = nums.length / 2 - 1; i >= 0; i--) {
+            siftDown(nums, nums.length, i);
+        }
+        // Extract the largest element from the heap and repeat for n-1 rounds
+        for (int i = nums.length - 1; i > 0; i--) {
+            // Swap the root node with the rightmost leaf node (swap the first element with the last element)
+            int tmp = nums[0];
+            nums[0] = nums[i];
+            nums[i] = tmp;
+            // Start heapifying the root node, from top to bottom
+            siftDown(nums, i, 0);
+        }
+    }
+
+    public static void main(String[] args) {
+        int[] nums = { 4, 1, 3, 1, 5, 2 };
+        heapSort(nums);
+        System.out.println("After heap sort, nums = " + Arrays.toString(nums));
+    }
+}

en/codes/java/chapter_sorting/insertion_sort.java

@@ -0,0 +1,31 @@
+/**
+ * File: insertion_sort.java
+ * Created Time: 2022-11-25
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_sorting;
+
+import java.util.*;
+
+public class insertion_sort {
+    /* Insertion sort */
+    static void insertionSort(int[] nums) {
+        // Outer loop: sorted range is [0, i-1]
+        for (int i = 1; i < nums.length; i++) {
+            int base = nums[i], j = i - 1;
+            // Inner loop: insert base into the correct position within the sorted range [0, i-1]
+            while (j >= 0 && nums[j] > base) {
+                nums[j + 1] = nums[j]; // Move nums[j] to the right by one position
+                j--;
+            }
+            nums[j + 1] = base;        // Assign base to the correct position
+        }
+    }
+
+    public static void main(String[] args) {
+        int[] nums = { 4, 1, 3, 1, 5, 2 };
+        insertionSort(nums);
+        System.out.println("After insertion sort, nums = " + Arrays.toString(nums));
+    }
+}

en/codes/java/chapter_sorting/merge_sort.java

@@ -0,0 +1,58 @@
+/**
+ * File: merge_sort.java
+ * Created Time: 2022-11-25
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_sorting;
+
+import java.util.*;
+
+public class merge_sort {
+    /* Merge left subarray and right subarray */
+    static void merge(int[] nums, int left, int mid, int right) {
+        // Left subarray interval is [left, mid], right subarray interval is [mid+1, right]
+        // Create a temporary array tmp to store the merged results
+        int[] tmp = new int[right - left + 1];
+        // Initialize the start indices of the left and right subarrays
+        int i = left, j = mid + 1, k = 0;
+        // While both subarrays still have elements, compare and copy the smaller element into the temporary array
+        while (i <= mid && j <= right) {
+            if (nums[i] <= nums[j])
+                tmp[k++] = nums[i++];
+            else
+                tmp[k++] = nums[j++];
+        }
+        // Copy the remaining elements of the left and right subarrays into the temporary array
+        while (i <= mid) {
+            tmp[k++] = nums[i++];
+        }
+        while (j <= right) {
+            tmp[k++] = nums[j++];
+        }
+        // Copy the elements from the temporary array tmp back to the original array nums at the corresponding interval
+        for (k = 0; k < tmp.length; k++) {
+            nums[left + k] = tmp[k];
+        }
+    }
+
+    /* Merge sort */
+    static void mergeSort(int[] nums, int left, int right) {
+        // Termination condition
+        if (left >= right)
+            return; // Terminate recursion when subarray length is 1
+        // Partition stage
+        int mid = (left + right) / 2; // Calculate midpoint
+        mergeSort(nums, left, mid); // Recursively process the left subarray
+        mergeSort(nums, mid + 1, right); // Recursively process the right subarray
+        // Merge stage
+        merge(nums, left, mid, right);
+    }
+
+    public static void main(String[] args) {
+        /* Merge sort */
+        int[] nums = { 7, 3, 2, 6, 0, 1, 5, 4 };
+        mergeSort(nums, 0, nums.length - 1);
+        System.out.println("After merge sort, nums = " + Arrays.toString(nums));
+    }
+}

en/codes/java/chapter_sorting/quick_sort.java

@@ -0,0 +1,158 @@
+/**
+ * File: quick_sort.java
+ * Created Time: 2022-11-25
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_sorting;
+
+import java.util.*;
+
+/* Quick sort class */
+class QuickSort {
+    /* Swap elements */
+    static void swap(int[] nums, int i, int j) {
+        int tmp = nums[i];
+        nums[i] = nums[j];
+        nums[j] = tmp;
+    }
+
+    /* Partition */
+    static int partition(int[] nums, int left, int right) {
+        // Use nums[left] as the pivot
+        int i = left, j = right;
+        while (i < j) {
+            while (i < j && nums[j] >= nums[left])
+                j--;          // Search from right to left for the first element smaller than the pivot
+            while (i < j && nums[i] <= nums[left])
+                i++;          // Search from left to right for the first element greater than the pivot
+            swap(nums, i, j); // Swap these two elements
+        }
+        swap(nums, i, left);  // Swap the pivot to the boundary between the two subarrays
+        return i;             // Return the index of the pivot
+    }
+
+    /* Quick sort */
+    public static void quickSort(int[] nums, int left, int right) {
+        // Terminate recursion when subarray length is 1
+        if (left >= right)
+            return;
+        // Partition
+        int pivot = partition(nums, left, right);
+        // Recursively process the left subarray and right subarray
+        quickSort(nums, left, pivot - 1);
+        quickSort(nums, pivot + 1, right);
+    }
+}
+
+/* Quick sort class (median pivot optimization) */
+class QuickSortMedian {
+    /* Swap elements */
+    static void swap(int[] nums, int i, int j) {
+        int tmp = nums[i];
+        nums[i] = nums[j];
+        nums[j] = tmp;
+    }
+
+    /* Select the median of three candidate elements */
+    static int medianThree(int[] nums, int left, int mid, int right) {
+        int l = nums[left], m = nums[mid], r = nums[right];
+        if ((l <= m && m <= r) || (r <= m && m <= l))
+            return mid; // m is between l and r
+        if ((m <= l && l <= r) || (r <= l && l <= m))
+            return left; // l is between m and r
+        return right;
+    }
+
+    /* Partition (median of three) */
+    static int partition(int[] nums, int left, int right) {
+        // Select the median of three candidate elements
+        int med = medianThree(nums, left, (left + right) / 2, right);
+        // Swap the median to the array's leftmost position
+        swap(nums, left, med);
+        // Use nums[left] as the pivot
+        int i = left, j = right;
+        while (i < j) {
+            while (i < j && nums[j] >= nums[left])
+                j--;          // Search from right to left for the first element smaller than the pivot
+            while (i < j && nums[i] <= nums[left])
+                i++;          // Search from left to right for the first element greater than the pivot
+            swap(nums, i, j); // Swap these two elements
+        }
+        swap(nums, i, left);  // Swap the pivot to the boundary between the two subarrays
+        return i;             // Return the index of the pivot
+    }
+
+    /* Quick sort */
+    public static void quickSort(int[] nums, int left, int right) {
+        // Terminate recursion when subarray length is 1
+        if (left >= right)
+            return;
+        // Partition
+        int pivot = partition(nums, left, right);
+        // Recursively process the left subarray and right subarray
+        quickSort(nums, left, pivot - 1);
+        quickSort(nums, pivot + 1, right);
+    }
+}
+
+/* Quick sort class (tail recursion optimization) */
+class QuickSortTailCall {
+    /* Swap elements */
+    static void swap(int[] nums, int i, int j) {
+        int tmp = nums[i];
+        nums[i] = nums[j];
+        nums[j] = tmp;
+    }
+
+    /* Partition */
+    static int partition(int[] nums, int left, int right) {
+        // Use nums[left] as the pivot
+        int i = left, j = right;
+        while (i < j) {
+            while (i < j && nums[j] >= nums[left])
+                j--;          // Search from right to left for the first element smaller than the pivot
+            while (i < j && nums[i] <= nums[left])
+                i++;          // Search from left to right for the first element greater than the pivot
+            swap(nums, i, j); // Swap these two elements
+        }
+        swap(nums, i, left);  // Swap the pivot to the boundary between the two subarrays
+        return i;             // Return the index of the pivot
+    }
+
+    /* Quick sort (tail recursion optimization) */
+    public static void quickSort(int[] nums, int left, int right) {
+        // Terminate when subarray length is 1
+        while (left < right) {
+            // Partition operation
+            int pivot = partition(nums, left, right);
+            // Perform quick sort on the shorter of the two subarrays
+            if (pivot - left < right - pivot) {
+                quickSort(nums, left, pivot - 1); // Recursively sort the left subarray
+                left = pivot + 1; // Remaining unsorted interval is [pivot + 1, right]
+            } else {
+                quickSort(nums, pivot + 1, right); // Recursively sort the right subarray
+                right = pivot - 1; // Remaining unsorted interval is [left, pivot - 1]
+            }
+        }
+    }
+}
+
+public class quick_sort {
+    public static void main(String[] args) {
+        /* Quick sort */
+        int[] nums = { 2, 4, 1, 0, 3, 5 };
+        QuickSort.quickSort(nums, 0, nums.length - 1);
+        System.out.println("After quick sort, nums = " + Arrays.toString(nums));
+
+        /* Quick sort (median pivot optimization) */
+        int[] nums1 = { 2, 4, 1, 0, 3, 5 };
+        QuickSortMedian.quickSort(nums1, 0, nums1.length - 1);
+        System.out.println("After quick sort with median pivot optimization, nums1 = " + Arrays.toString(nums1));
+
+        /* Quick sort (tail recursion optimization) */
+        int[] nums2 = { 2, 4, 1, 0, 3, 5 };
+        QuickSortTailCall.quickSort(nums2, 0, nums2.length - 1);
+        System.out.println("After quick sort with tail recursion optimization, nums2 = " + Arrays.toString(nums2));
+    }
+}

en/codes/java/chapter_sorting/radix_sort.java

@@ -0,0 +1,69 @@
+/**
+ * File: radix_sort.java
+ * Created Time: 2023-01-17
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_sorting;
+
+import java.util.*;
+
+public class radix_sort {
+    /* Get the k-th digit of element num, where exp = 10^(k-1) */
+    static int digit(int num, int exp) {
+        // Passing exp instead of k can avoid repeated expensive exponentiation here
+        return (num / exp) % 10;
+    }
+
+    /* Counting sort (based on nums k-th digit) */
+    static void countingSortDigit(int[] nums, int exp) {
+        // Decimal digit range is 0~9, therefore need a bucket array of length 10
+        int[] counter = new int[10];
+        int n = nums.length;
+        // Count the occurrence of digits 0~9
+        for (int i = 0; i < n; i++) {
+            int d = digit(nums[i], exp); // Get the k-th digit of nums[i], noted as d
+            counter[d]++;                // Count the occurrence of digit d
+        }
+        // Calculate prefix sum, converting "occurrence count" into "array index"
+        for (int i = 1; i < 10; i++) {
+            counter[i] += counter[i - 1];
+        }
+        // Traverse in reverse, based on bucket statistics, place each element into res
+        int[] res = new int[n];
+        for (int i = n - 1; i >= 0; i--) {
+            int d = digit(nums[i], exp);
+            int j = counter[d] - 1; // Get the index j for d in the array
+            res[j] = nums[i];       // Place the current element at index j
+            counter[d]--;           // Decrease the count of d by 1
+        }
+        // Use result to overwrite the original array nums
+        for (int i = 0; i < n; i++)
+            nums[i] = res[i];
+    }
+
+    /* Radix sort */
+    static void radixSort(int[] nums) {
+        // Get the maximum element of the array, used to determine the maximum number of digits
+        int m = Integer.MIN_VALUE;
+        for (int num : nums)
+            if (num > m)
+                m = num;
+        // Traverse from the lowest to the highest digit
+        for (int exp = 1; exp <= m; exp *= 10) {
+            // Perform counting sort on the k-th digit of array elements
+            // k = 1 -> exp = 1
+            // k = 2 -> exp = 10
+            // i.e., exp = 10^(k-1)
+            countingSortDigit(nums, exp);
+        }
+    }
+
+    public static void main(String[] args) {
+        // Radix sort
+        int[] nums = { 10546151, 35663510, 42865989, 34862445, 81883077,
+                       88906420, 72429244, 30524779, 82060337, 63832996 };
+        radixSort(nums);
+        System.out.println("After radix sort, nums = " + Arrays.toString(nums));
+    }
+}

en/codes/java/chapter_sorting/selection_sort.java

@@ -0,0 +1,35 @@
+/**
+ * File: selection_sort.java
+ * Created Time: 2023-05-23
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_sorting;
+
+import java.util.Arrays;
+
+public class selection_sort {
+    /* Selection sort */
+    public static void selectionSort(int[] nums) {
+        int n = nums.length;
+        // Outer loop: unsorted range is [i, n-1]
+        for (int i = 0; i < n - 1; i++) {
+            // Inner loop: find the smallest element within the unsorted range
+            int k = i;
+            for (int j = i + 1; j < n; j++) {
+                if (nums[j] < nums[k])
+                    k = j; // Record the index of the smallest element
+            }
+            // Swap the smallest element with the first element of the unsorted range
+            int temp = nums[i];
+            nums[i] = nums[k];
+            nums[k] = temp;
+        }
+    }
+
+    public static void main(String[] args) {
+        int[] nums = { 4, 1, 3, 1, 5, 2 };
+        selectionSort(nums);
+        System.out.println("After selection sort, nums = " + Arrays.toString(nums));
+    }
+}

en/codes/java/chapter_stack_and_queue/array_deque.java

@@ -0,0 +1,151 @@
+/**
+ * File: array_deque.java
+ * Created Time: 2023-02-16
+ * Author: krahets (krahets@163.com), FangYuan33 (374072213@qq.com)
+ */
+
+package chapter_stack_and_queue;
+
+import java.util.*;
+
+/* Double-ended queue class based on circular array */
+class ArrayDeque {
+    private int[] nums; // Array used to store elements of the double-ended queue
+    private int front; // Front pointer, pointing to the front element
+    private int queSize; // Length of the double-ended queue
+
+    /* Constructor */
+    public ArrayDeque(int capacity) {
+        this.nums = new int[capacity];
+        front = queSize = 0;
+    }
+
+    /* Get the capacity of the double-ended queue */
+    public int capacity() {
+        return nums.length;
+    }
+
+    /* Get the length of the double-ended queue */
+    public int size() {
+        return queSize;
+    }
+
+    /* Determine if the double-ended queue is empty */
+    public boolean isEmpty() {
+        return queSize == 0;
+    }
+
+    /* Calculate circular array index */
+    private int index(int i) {
+        // Implement circular array by modulo operation
+        // When i exceeds the tail of the array, return to the head
+        // When i exceeds the head of the array, return to the tail
+        return (i + capacity()) % capacity();
+    }
+
+    /* Front enqueue */
+    public void pushFirst(int num) {
+        if (queSize == capacity()) {
+            System.out.println("Double-ended queue is full");
+            return;
+        }
+        // Move the front pointer one position to the left
+        // Implement front crossing the head of the array to return to the tail by modulo operation
+        front = index(front - 1);
+        // Add num to the front
+        nums[front] = num;
+        queSize++;
+    }
+
+    /* Rear enqueue */
+    public void pushLast(int num) {
+        if (queSize == capacity()) {
+            System.out.println("Double-ended queue is full");
+            return;
+        }
+        // Calculate rear pointer, pointing to rear index + 1
+        int rear = index(front + queSize);
+        // Add num to the rear
+        nums[rear] = num;
+        queSize++;
+    }
+
+    /* Front dequeue */
+    public int popFirst() {
+        int num = peekFirst();
+        // Move front pointer one position backward
+        front = index(front + 1);
+        queSize--;
+        return num;
+    }
+
+    /* Rear dequeue */
+    public int popLast() {
+        int num = peekLast();
+        queSize--;
+        return num;
+    }
+
+    /* Access front element */
+    public int peekFirst() {
+        if (isEmpty())
+            throw new IndexOutOfBoundsException();
+        return nums[front];
+    }
+
+    /* Access rear element */
+    public int peekLast() {
+        if (isEmpty())
+            throw new IndexOutOfBoundsException();
+        // Calculate rear element index
+        int last = index(front + queSize - 1);
+        return nums[last];
+    }
+
+    /* Return array for printing */
+    public int[] toArray() {
+        // Only convert elements within valid length range
+        int[] res = new int[queSize];
+        for (int i = 0, j = front; i < queSize; i++, j++) {
+            res[i] = nums[index(j)];
+        }
+        return res;
+    }
+}
+
+public class array_deque {
+    public static void main(String[] args) {
+        /* Initialize double-ended queue */
+        ArrayDeque deque = new ArrayDeque(10);
+        deque.pushLast(3);
+        deque.pushLast(2);
+        deque.pushLast(5);
+        System.out.println("Double-ended queue deque = " + Arrays.toString(deque.toArray()));
+
+        /* Access element */
+        int peekFirst = deque.peekFirst();
+        System.out.println("Front element peekFirst = " + peekFirst);
+        int peekLast = deque.peekLast();
+        System.out.println("Back element peekLast = " + peekLast);
+
+        /* Element enqueue */
+        deque.pushLast(4);
+        System.out.println("Element 4 enqueued at the tail, deque = " + Arrays.toString(deque.toArray()));
+        deque.pushFirst(1);
+        System.out.println("Element 1 enqueued at the head, deque = " + Arrays.toString(deque.toArray()));
+
+        /* Element dequeue */
+        int popLast = deque.popLast();
+        System.out.println("Deque tail element = " + popLast + ", after dequeuing from the tail" + Arrays.toString(deque.toArray()));
+        int popFirst = deque.popFirst();
+        System.out.println("Deque front element = " + popFirst + ", after dequeuing from the front" + Arrays.toString(deque.toArray()));
+
+        /* Get the length of the double-ended queue */
+        int size = deque.size();
+        System.out.println("Length of the double-ended queue size = " + size);
+
+        /* Determine if the double-ended queue is empty */
+        boolean isEmpty = deque.isEmpty();
+        System.out.println("Is the double-ended queue empty = " + isEmpty);
+    }
+}

en/codes/java/chapter_stack_and_queue/array_queue.java

@@ -0,0 +1,115 @@
+/**
+ * File: array_queue.java
+ * Created Time: 2022-11-25
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_stack_and_queue;
+
+import java.util.*;
+
+/* Queue class based on circular array */
+class ArrayQueue {
+    private int[] nums; // Array for storing queue elements
+    private int front; // Front pointer, pointing to the front element
+    private int queSize; // Queue length
+
+    public ArrayQueue(int capacity) {
+        nums = new int[capacity];
+        front = queSize = 0;
+    }
+
+    /* Get the capacity of the queue */
+    public int capacity() {
+        return nums.length;
+    }
+
+    /* Get the length of the queue */
+    public int size() {
+        return queSize;
+    }
+
+    /* Determine if the queue is empty */
+    public boolean isEmpty() {
+        return queSize == 0;
+    }
+
+    /* Enqueue */
+    public void push(int num) {
+        if (queSize == capacity()) {
+            System.out.println("Queue is full");
+            return;
+        }
+        // Calculate rear pointer, pointing to rear index + 1
+        // Use modulo operation to wrap the rear pointer from the end of the array back to the start
+        int rear = (front + queSize) % capacity();
+        // Add num to the rear
+        nums[rear] = num;
+        queSize++;
+    }
+
+    /* Dequeue */
+    public int pop() {
+        int num = peek();
+        // Move front pointer one position backward, returning to the head of the array if it exceeds the tail
+        front = (front + 1) % capacity();
+        queSize--;
+        return num;
+    }
+
+    /* Access front element */
+    public int peek() {
+        if (isEmpty())
+            throw new IndexOutOfBoundsException();
+        return nums[front];
+    }
+
+    /* Return array */
+    public int[] toArray() {
+        // Only convert elements within valid length range
+        int[] res = new int[queSize];
+        for (int i = 0, j = front; i < queSize; i++, j++) {
+            res[i] = nums[j % capacity()];
+        }
+        return res;
+    }
+}
+
+public class array_queue {
+    public static void main(String[] args) {
+        /* Initialize queue */
+        int capacity = 10;
+        ArrayQueue queue = new ArrayQueue(capacity);
+
+        /* Element enqueue */
+        queue.push(1);
+        queue.push(3);
+        queue.push(2);
+        queue.push(5);
+        queue.push(4);
+        System.out.println("Queue queue = " + Arrays.toString(queue.toArray()));
+
+        /* Access front element */
+        int peek = queue.peek();
+        System.out.println("Front element peek = " + peek);
+
+        /* Element dequeue */
+        int pop = queue.pop();
+        System.out.println("Dequeued element = " + pop + ", after dequeuing" + Arrays.toString(queue.toArray()));
+
+        /* Get the length of the queue */
+        int size = queue.size();
+        System.out.println("Length of the queue size = " + size);
+
+        /* Determine if the queue is empty */
+        boolean isEmpty = queue.isEmpty();
+        System.out.println("Is the queue empty = " + isEmpty);
+
+        /* Test circular array */
+        for (int i = 0; i < 10; i++) {
+            queue.push(i);
+            queue.pop();
+            System.out.println("After the " + i + "th round of enqueueing + dequeuing, queue = " + Arrays.toString(queue.toArray()));
+        }
+    }
+}

en/codes/java/chapter_stack_and_queue/array_stack.java

@@ -0,0 +1,84 @@
+/**
+ * File: array_stack.java
+ * Created Time: 2022-11-25
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_stack_and_queue;
+
+import java.util.*;
+
+/* Stack class based on array */
+class ArrayStack {
+    private ArrayList<Integer> stack;
+
+    public ArrayStack() {
+        // Initialize the list (dynamic array)
+        stack = new ArrayList<>();
+    }
+
+    /* Get the length of the stack */
+    public int size() {
+        return stack.size();
+    }
+
+    /* Determine if the stack is empty */
+    public boolean isEmpty() {
+        return size() == 0;
+    }
+
+    /* Push */
+    public void push(int num) {
+        stack.add(num);
+    }
+
+    /* Pop */
+    public int pop() {
+        if (isEmpty())
+            throw new IndexOutOfBoundsException();
+        return stack.remove(size() - 1);
+    }
+
+    /* Access stack top element */
+    public int peek() {
+        if (isEmpty())
+            throw new IndexOutOfBoundsException();
+        return stack.get(size() - 1);
+    }
+
+    /* Convert the List to Array and return */
+    public Object[] toArray() {
+        return stack.toArray();
+    }
+}
+
+public class array_stack {
+    public static void main(String[] args) {
+        /* Initialize stack */
+        ArrayStack stack = new ArrayStack();
+
+        /* Element push */
+        stack.push(1);
+        stack.push(3);
+        stack.push(2);
+        stack.push(5);
+        stack.push(4);
+        System.out.println("Stack stack = " + Arrays.toString(stack.toArray()));
+
+        /* Access stack top element */
+        int peek = stack.peek();
+        System.out.println("Top element peek = " + peek);
+
+        /* Element pop */
+        int pop = stack.pop();
+        System.out.println("Popped element = " + pop + ", after popping" + Arrays.toString(stack.toArray()));
+
+        /* Get the length of the stack */
+        int size = stack.size();
+        System.out.println("Length of the stack size = " + size);
+
+        /* Determine if it's empty */
+        boolean isEmpty = stack.isEmpty();
+        System.out.println("Is the stack empty = " + isEmpty);
+    }
+}

en/codes/java/chapter_stack_and_queue/deque.java

@@ -0,0 +1,46 @@
+/**
+ * File: deque.java
+ * Created Time: 2022-11-25
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_stack_and_queue;
+
+import java.util.*;
+
+public class deque {
+    public static void main(String[] args) {
+        /* Initialize double-ended queue */
+        Deque<Integer> deque = new LinkedList<>();
+        deque.offerLast(3);
+        deque.offerLast(2);
+        deque.offerLast(5);
+        System.out.println("Double-ended queue deque = " + deque);
+
+        /* Access element */
+        int peekFirst = deque.peekFirst();
+        System.out.println("Front element peekFirst = " + peekFirst);
+        int peekLast = deque.peekLast();
+        System.out.println("Back element peekLast = " + peekLast);
+
+        /* Element enqueue */
+        deque.offerLast(4);
+        System.out.println("Element 4 enqueued at the tail, deque = " + deque);
+        deque.offerFirst(1);
+        System.out.println("Element 1 enqueued at the head, deque = " + deque);
+
+        /* Element dequeue */
+        int popLast = deque.pollLast();
+        System.out.println("Deque tail element = " + popLast + ", after dequeuing from the tail" + deque);
+        int popFirst = deque.pollFirst();
+        System.out.println("Deque front element = " + popFirst + ", after dequeuing from the front" + deque);
+
+        /* Get the length of the double-ended queue */
+        int size = deque.size();
+        System.out.println("Length of the double-ended queue size = " + size);
+
+        /* Determine if the double-ended queue is empty */
+        boolean isEmpty = deque.isEmpty();
+        System.out.println("Is the double-ended queue empty = " + isEmpty);
+    }
+}

en/codes/java/chapter_stack_and_queue/linkedlist_deque.java

@@ -0,0 +1,175 @@
+/**
+ * File: linkedlist_deque.java
+ * Created Time: 2023-01-20
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_stack_and_queue;
+
+import java.util.*;
+
+/* Double-linked list node */
+class ListNode {
+    int val; // Node value
+    ListNode next; // Reference to the next node
+    ListNode prev; // Reference to predecessor node
+
+    ListNode(int val) {
+        this.val = val;
+        prev = next = null;
+    }
+}
+
+/* Double-ended queue class based on double-linked list */
+class LinkedListDeque {
+    private ListNode front, rear; // Front node front, back node rear
+    private int queSize = 0; // Length of the double-ended queue
+
+    public LinkedListDeque() {
+        front = rear = null;
+    }
+
+    /* Get the length of the double-ended queue */
+    public int size() {
+        return queSize;
+    }
+
+    /* Determine if the double-ended queue is empty */
+    public boolean isEmpty() {
+        return size() == 0;
+    }
+
+    /* Enqueue operation */
+    private void push(int num, boolean isFront) {
+        ListNode node = new ListNode(num);
+        // If the list is empty, make front and rear both point to node
+        if (isEmpty())
+            front = rear = node;
+        // Front enqueue operation
+        else if (isFront) {
+            // Add node to the head of the list
+            front.prev = node;
+            node.next = front;
+            front = node; // Update head node
+        // Rear enqueue operation
+        } else {
+            // Add node to the tail of the list
+            rear.next = node;
+            node.prev = rear;
+            rear = node; // Update tail node
+        }
+        queSize++; // Update queue length
+    }
+
+    /* Front enqueue */
+    public void pushFirst(int num) {
+        push(num, true);
+    }
+
+    /* Rear enqueue */
+    public void pushLast(int num) {
+        push(num, false);
+    }
+
+    /* Dequeue operation */
+    private int pop(boolean isFront) {
+        if (isEmpty())
+            throw new IndexOutOfBoundsException();
+        int val;
+        // Front dequeue operation
+        if (isFront) {
+            val = front.val; // Temporarily store the head node value
+            // Remove head node
+            ListNode fNext = front.next;
+            if (fNext != null) {
+                fNext.prev = null;
+                front.next = null;
+            }
+            front = fNext; // Update head node
+        // Rear dequeue operation
+        } else {
+            val = rear.val; // Temporarily store the tail node value
+            // Remove tail node
+            ListNode rPrev = rear.prev;
+            if (rPrev != null) {
+                rPrev.next = null;
+                rear.prev = null;
+            }
+            rear = rPrev; // Update tail node
+        }
+        queSize--; // Update queue length
+        return val;
+    }
+
+    /* Front dequeue */
+    public int popFirst() {
+        return pop(true);
+    }
+
+    /* Rear dequeue */
+    public int popLast() {
+        return pop(false);
+    }
+
+    /* Access front element */
+    public int peekFirst() {
+        if (isEmpty())
+            throw new IndexOutOfBoundsException();
+        return front.val;
+    }
+
+    /* Access rear element */
+    public int peekLast() {
+        if (isEmpty())
+            throw new IndexOutOfBoundsException();
+        return rear.val;
+    }
+
+    /* Return array for printing */
+    public int[] toArray() {
+        ListNode node = front;
+        int[] res = new int[size()];
+        for (int i = 0; i < res.length; i++) {
+            res[i] = node.val;
+            node = node.next;
+        }
+        return res;
+    }
+}
+
+public class linkedlist_deque {
+    public static void main(String[] args) {
+        /* Initialize double-ended queue */
+        LinkedListDeque deque = new LinkedListDeque();
+        deque.pushLast(3);
+        deque.pushLast(2);
+        deque.pushLast(5);
+        System.out.println("Double-ended queue deque = " + Arrays.toString(deque.toArray()));
+
+        /* Access element */
+        int peekFirst = deque.peekFirst();
+        System.out.println("Front element peekFirst = " + peekFirst);
+        int peekLast = deque.peekLast();
+        System.out.println("Back element peekLast = " + peekLast);
+
+        /* Element enqueue */
+        deque.pushLast(4);
+        System.out.println("Element 4 enqueued at the tail, deque = " + Arrays.toString(deque.toArray()));
+        deque.pushFirst(1);
+        System.out.println("Element 1 enqueued at the head, deque = " + Arrays.toString(deque.toArray()));
+
+        /* Element dequeue */
+        int popLast = deque.popLast();
+        System.out.println("Deque tail element = " + popLast + ", after dequeuing from the tail" + Arrays.toString(deque.toArray()));
+        int popFirst = deque.popFirst();
+        System.out.println("Deque front element = " + popFirst + ", after dequeuing from the front" + Arrays.toString(deque.toArray()));
+
+        /* Get the length of the double-ended queue */
+        int size = deque.size();
+        System.out.println("Length of the double-ended queue size = " + size);
+
+        /* Determine if the double-ended queue is empty */
+        boolean isEmpty = deque.isEmpty();
+        System.out.println("Is the double-ended queue empty = " + isEmpty);
+    }
+}

en/codes/java/chapter_stack_and_queue/linkedlist_queue.java

@@ -0,0 +1,104 @@
+/**
+ * File: linkedlist_queue.java
+ * Created Time: 2022-11-25
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_stack_and_queue;
+
+import java.util.*;
+
+/* Queue class based on linked list */
+class LinkedListQueue {
+    private ListNode front, rear; // Front node front, back node rear
+    private int queSize = 0;
+
+    public LinkedListQueue() {
+        front = null;
+        rear = null;
+    }
+
+    /* Get the length of the queue */
+    public int size() {
+        return queSize;
+    }
+
+    /* Determine if the queue is empty */
+    public boolean isEmpty() {
+        return size() == 0;
+    }
+
+    /* Enqueue */
+    public void push(int num) {
+        // Add num behind the tail node
+        ListNode node = new ListNode(num);
+        // If the queue is empty, make the head and tail nodes both point to that node
+        if (front == null) {
+            front = node;
+            rear = node;
+        // If the queue is not empty, add that node behind the tail node
+        } else {
+            rear.next = node;
+            rear = node;
+        }
+        queSize++;
+    }
+
+    /* Dequeue */
+    public int pop() {
+        int num = peek();
+        // Remove head node
+        front = front.next;
+        queSize--;
+        return num;
+    }
+
+    /* Access front element */
+    public int peek() {
+        if (isEmpty())
+            throw new IndexOutOfBoundsException();
+        return front.val;
+    }
+
+    /* Convert the linked list to Array and return */
+    public int[] toArray() {
+        ListNode node = front;
+        int[] res = new int[size()];
+        for (int i = 0; i < res.length; i++) {
+            res[i] = node.val;
+            node = node.next;
+        }
+        return res;
+    }
+}
+
+public class linkedlist_queue {
+    public static void main(String[] args) {
+        /* Initialize queue */
+        LinkedListQueue queue = new LinkedListQueue();
+
+        /* Element enqueue */
+        queue.push(1);
+        queue.push(3);
+        queue.push(2);
+        queue.push(5);
+        queue.push(4);
+        System.out.println("Queue queue = " + Arrays.toString(queue.toArray()));
+
+        /* Access front element */
+        int peek = queue.peek();
+        System.out.println("Front element peek = " + peek);
+
+        /* Element dequeue */
+        int pop = queue.pop();
+        System.out.println("Dequeued element = " + pop + ", after dequeuing" + Arrays.toString(queue.toArray()));
+
+        /* Get the length of the queue */
+        int size = queue.size();
+        System.out.println("Length of the queue size = " + size);
+
+        /* Determine if the queue is empty */
+        boolean isEmpty = queue.isEmpty();
+        System.out.println("Is the queue empty = " + isEmpty);
+    }
+}

en/codes/java/chapter_stack_and_queue/linkedlist_stack.java

@@ -0,0 +1,95 @@
+/**
+ * File: linkedlist_stack.java
+ * Created Time: 2022-11-25
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_stack_and_queue;
+
+import java.util.*;
+import utils.*;
+
+/* Stack class based on linked list */
+class LinkedListStack {
+    private ListNode stackPeek; // Use the head node as the top of the stack
+    private int stkSize = 0; // Length of the stack
+
+    public LinkedListStack() {
+        stackPeek = null;
+    }
+
+    /* Get the length of the stack */
+    public int size() {
+        return stkSize;
+    }
+
+    /* Determine if the stack is empty */
+    public boolean isEmpty() {
+        return size() == 0;
+    }
+
+    /* Push */
+    public void push(int num) {
+        ListNode node = new ListNode(num);
+        node.next = stackPeek;
+        stackPeek = node;
+        stkSize++;
+    }
+
+    /* Pop */
+    public int pop() {
+        int num = peek();
+        stackPeek = stackPeek.next;
+        stkSize--;
+        return num;
+    }
+
+    /* Access stack top element */
+    public int peek() {
+        if (isEmpty())
+            throw new IndexOutOfBoundsException();
+        return stackPeek.val;
+    }
+
+    /* Convert the List to Array and return */
+    public int[] toArray() {
+        ListNode node = stackPeek;
+        int[] res = new int[size()];
+        for (int i = res.length - 1; i >= 0; i--) {
+            res[i] = node.val;
+            node = node.next;
+        }
+        return res;
+    }
+}
+
+public class linkedlist_stack {
+    public static void main(String[] args) {
+        /* Initialize stack */
+        LinkedListStack stack = new LinkedListStack();
+
+        /* Element push */
+        stack.push(1);
+        stack.push(3);
+        stack.push(2);
+        stack.push(5);
+        stack.push(4);
+        System.out.println("Stack stack = " + Arrays.toString(stack.toArray()));
+
+        /* Access stack top element */
+        int peek = stack.peek();
+        System.out.println("Top element peek = " + peek);
+
+        /* Element pop */
+        int pop = stack.pop();
+        System.out.println("Popped element = " + pop + ", after popping" + Arrays.toString(stack.toArray()));
+
+        /* Get the length of the stack */
+        int size = stack.size();
+        System.out.println("Length of the stack size = " + size);
+
+        /* Determine if it's empty */
+        boolean isEmpty = stack.isEmpty();
+        System.out.println("Is the stack empty = " + isEmpty);
+    }
+}

en/codes/java/chapter_stack_and_queue/queue.java

@@ -0,0 +1,40 @@
+/**
+ * File: queue.java
+ * Created Time: 2022-11-25
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_stack_and_queue;
+
+import java.util.*;
+
+public class queue {
+    public static void main(String[] args) {
+        /* Initialize queue */
+        Queue<Integer> queue = new LinkedList<>();
+
+        /* Element enqueue */
+        queue.offer(1);
+        queue.offer(3);
+        queue.offer(2);
+        queue.offer(5);
+        queue.offer(4);
+        System.out.println("Queue queue = " + queue);
+
+        /* Access front element */
+        int peek = queue.peek();
+        System.out.println("Front element peek = " + peek);
+
+        /* Element dequeue */
+        int pop = queue.poll();
+        System.out.println("Dequeued element = " + pop + ", after dequeuing" + queue);
+
+        /* Get the length of the queue */
+        int size = queue.size();
+        System.out.println("Length of the queue size = " + size);
+
+        /* Determine if the queue is empty */
+        boolean isEmpty = queue.isEmpty();
+        System.out.println("Is the queue empty = " + isEmpty);
+    }
+}

en/codes/java/chapter_stack_and_queue/stack.java

@@ -0,0 +1,40 @@
+/**
+ * File: stack.java
+ * Created Time: 2022-11-25
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_stack_and_queue;
+
+import java.util.*;
+
+public class stack {
+    public static void main(String[] args) {
+        /* Initialize stack */
+        Stack<Integer> stack = new Stack<>();
+
+        /* Element push */
+        stack.push(1);
+        stack.push(3);
+        stack.push(2);
+        stack.push(5);
+        stack.push(4);
+        System.out.println("Stack stack = " + stack);
+
+        /* Access stack top element */
+        int peek = stack.peek();
+        System.out.println("Top element peek = " + peek);
+
+        /* Element pop */
+        int pop = stack.pop();
+        System.out.println("Popped element = " + pop + ", after popping" + stack);
+
+        /* Get the length of the stack */
+        int size = stack.size();
+        System.out.println("Length of the stack size = " + size);
+
+        /* Determine if it's empty */
+        boolean isEmpty = stack.isEmpty();
+        System.out.println("Is the stack empty = " + isEmpty);
+    }
+}

en/codes/java/chapter_tree/array_binary_tree.java

@@ -0,0 +1,136 @@
+/**
+ * File: array_binary_tree.java
+ * Created Time: 2023-07-19
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_tree;
+
+import utils.*;
+import java.util.*;
+
+/* Array-based binary tree class */
+class ArrayBinaryTree {
+    private List<Integer> tree;
+
+    /* Constructor */
+    public ArrayBinaryTree(List<Integer> arr) {
+        tree = new ArrayList<>(arr);
+    }
+
+    /* List capacity */
+    public int size() {
+        return tree.size();
+    }
+
+    /* Get the value of the node at index i */
+    public Integer val(int i) {
+        // If the index is out of bounds, return null, representing an empty spot
+        if (i < 0 || i >= size())
+            return null;
+        return tree.get(i);
+    }
+
+    /* Get the index of the left child of the node at index i */
+    public Integer left(int i) {
+        return 2 * i + 1;
+    }
+
+    /* Get the index of the right child of the node at index i */
+    public Integer right(int i) {
+        return 2 * i + 2;
+    }
+
+    /* Get the index of the parent of the node at index i */
+    public Integer parent(int i) {
+        return (i - 1) / 2;
+    }
+
+    /* Level-order traversal */
+    public List<Integer> levelOrder() {
+        List<Integer> res = new ArrayList<>();
+        // Traverse array
+        for (int i = 0; i < size(); i++) {
+            if (val(i) != null)
+                res.add(val(i));
+        }
+        return res;
+    }
+
+    /* Depth-first traversal */
+    private void dfs(Integer i, String order, List<Integer> res) {
+        // If it is an empty spot, return
+        if (val(i) == null)
+            return;
+        // Pre-order traversal
+        if ("pre".equals(order))
+            res.add(val(i));
+        dfs(left(i), order, res);
+        // In-order traversal
+        if ("in".equals(order))
+            res.add(val(i));
+        dfs(right(i), order, res);
+        // Post-order traversal
+        if ("post".equals(order))
+            res.add(val(i));
+    }
+
+    /* Pre-order traversal */
+    public List<Integer> preOrder() {
+        List<Integer> res = new ArrayList<>();
+        dfs(0, "pre", res);
+        return res;
+    }
+
+    /* In-order traversal */
+    public List<Integer> inOrder() {
+        List<Integer> res = new ArrayList<>();
+        dfs(0, "in", res);
+        return res;
+    }
+
+    /* Post-order traversal */
+    public List<Integer> postOrder() {
+        List<Integer> res = new ArrayList<>();
+        dfs(0, "post", res);
+        return res;
+    }
+}
+
+public class array_binary_tree {
+    public static void main(String[] args) {
+        // Initialize binary tree
+        // Use a specific function to convert an array into a binary tree
+        List<Integer> arr = Arrays.asList(1, 2, 3, 4, null, 6, 7, 8, 9, null, null, 12, null, null, 15);
+
+        TreeNode root = TreeNode.listToTree(arr);
+        System.out.println("\nInitialize binary tree\n");
+        System.out.println("Array representation of the binary tree:");
+        System.out.println(arr);
+        System.out.println("Linked list representation of the binary tree:");
+        PrintUtil.printTree(root);
+
+        // Array-based binary tree class
+        ArrayBinaryTree abt = new ArrayBinaryTree(arr);
+
+        // Access node
+        int i = 1;
+        Integer l = abt.left(i);
+        Integer r = abt.right(i);
+        Integer p = abt.parent(i);
+        System.out.println("\nThe current node's index is " + i + ", value = " + abt.val(i));
+        System.out.println("Its left child's index is " + l + ", value = " + (l == null ? "null" : abt.val(l)));
+        System.out.println("Its right child's index is " + r + ", value = " + (r == null ? "null" : abt.val(r)));
+        System.out.println("Its parent's index is " + p + ", value = " + (p == null ? "null" : abt.val(p)));
+
+        // Traverse tree
+        List<Integer> res = abt.levelOrder();
+        System.out.println("\nLevel-order traversal is:" + res);
+        res = abt.preOrder();
+        System.out.println("Pre-order traversal is:" + res);
+        res = abt.inOrder();
+        System.out.println("In-order traversal is:" + res);
+        res = abt.postOrder();
+        System.out.println("Post-order traversal is:" + res);
+    }
+}

en/codes/java/chapter_tree/avl_tree.java

@@ -0,0 +1,220 @@
+/**
+ * File: avl_tree.java
+ * Created Time: 2022-12-10
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_tree;
+
+import utils.*;
+
+/* AVL tree */
+class AVLTree {
+    TreeNode root; // Root node
+
+    /* Get node height */
+    public int height(TreeNode node) {
+        // Empty node height is -1, leaf node height is 0
+        return node == null ? -1 : node.height;
+    }
+
+    /* Update node height */
+    private void updateHeight(TreeNode node) {
+        // Node height equals the height of the tallest subtree + 1
+        node.height = Math.max(height(node.left), height(node.right)) + 1;
+    }
+
+    /* Get balance factor */
+    public int balanceFactor(TreeNode node) {
+        // Empty node balance factor is 0
+        if (node == null)
+            return 0;
+        // Node balance factor = left subtree height - right subtree height
+        return height(node.left) - height(node.right);
+    }
+
+    /* Right rotation operation */
+    private TreeNode rightRotate(TreeNode node) {
+        TreeNode child = node.left;
+        TreeNode grandChild = child.right;
+        // Rotate node to the right around child
+        child.right = node;
+        node.left = grandChild;
+        // Update node height
+        updateHeight(node);
+        updateHeight(child);
+        // Return the root of the subtree after rotation
+        return child;
+    }
+
+    /* Left rotation operation */
+    private TreeNode leftRotate(TreeNode node) {
+        TreeNode child = node.right;
+        TreeNode grandChild = child.left;
+        // Rotate node to the left around child
+        child.left = node;
+        node.right = grandChild;
+        // Update node height
+        updateHeight(node);
+        updateHeight(child);
+        // Return the root of the subtree after rotation
+        return child;
+    }
+
+    /* Perform rotation operation to restore balance to the subtree */
+    private TreeNode rotate(TreeNode node) {
+        // Get the balance factor of node
+        int balanceFactor = balanceFactor(node);
+        // Left-leaning tree
+        if (balanceFactor > 1) {
+            if (balanceFactor(node.left) >= 0) {
+                // Right rotation
+                return rightRotate(node);
+            } else {
+                // First left rotation then right rotation
+                node.left = leftRotate(node.left);
+                return rightRotate(node);
+            }
+        }
+        // Right-leaning tree
+        if (balanceFactor < -1) {
+            if (balanceFactor(node.right) <= 0) {
+                // Left rotation
+                return leftRotate(node);
+            } else {
+                // First right rotation then left rotation
+                node.right = rightRotate(node.right);
+                return leftRotate(node);
+            }
+        }
+        // Balanced tree, no rotation needed, return
+        return node;
+    }
+
+    /* Insert node */
+    public void insert(int val) {
+        root = insertHelper(root, val);
+    }
+
+    /* Recursively insert node (helper method) */
+    private TreeNode insertHelper(TreeNode node, int val) {
+        if (node == null)
+            return new TreeNode(val);
+        /* 1. Find insertion position and insert node */
+        if (val < node.val)
+            node.left = insertHelper(node.left, val);
+        else if (val > node.val)
+            node.right = insertHelper(node.right, val);
+        else
+            return node; // Do not insert duplicate nodes, return
+        updateHeight(node); // Update node height
+        /* 2. Perform rotation operation to restore balance to the subtree */
+        node = rotate(node);
+        // Return the root node of the subtree
+        return node;
+    }
+
+    /* Remove node */
+    public void remove(int val) {
+        root = removeHelper(root, val);
+    }
+
+    /* Recursively remove node (helper method) */
+    private TreeNode removeHelper(TreeNode node, int val) {
+        if (node == null)
+            return null;
+        /* 1. Find and remove the node */
+        if (val < node.val)
+            node.left = removeHelper(node.left, val);
+        else if (val > node.val)
+            node.right = removeHelper(node.right, val);
+        else {
+            if (node.left == null || node.right == null) {
+                TreeNode child = node.left != null ? node.left : node.right;
+                // Number of child nodes = 0, remove node and return
+                if (child == null)
+                    return null;
+                // Number of child nodes = 1, remove node
+                else
+                    node = child;
+            } else {
+                // Number of child nodes = 2, remove the next node in in-order traversal and replace the current node with it
+                TreeNode temp = node.right;
+                while (temp.left != null) {
+                    temp = temp.left;
+                }
+                node.right = removeHelper(node.right, temp.val);
+                node.val = temp.val;
+            }
+        }
+        updateHeight(node); // Update node height
+        /* 2. Perform rotation operation to restore balance to the subtree */
+        node = rotate(node);
+        // Return the root node of the subtree
+        return node;
+    }
+
+    /* Search node */
+    public TreeNode search(int val) {
+        TreeNode cur = root;
+        // Loop find, break after passing leaf nodes
+        while (cur != null) {
+            // Target node is in cur's right subtree
+            if (cur.val < val)
+                cur = cur.right;
+            // Target node is in cur's left subtree
+            else if (cur.val > val)
+                cur = cur.left;
+            // Found target node, break loop
+            else
+                break;
+        }
+        // Return target node
+        return cur;
+    }
+}
+
+public class avl_tree {
+    static void testInsert(AVLTree tree, int val) {
+        tree.insert(val);
+        System.out.println("\nAfter inserting node " + val + ", the AVL tree is ");
+        PrintUtil.printTree(tree.root);
+    }
+
+    static void testRemove(AVLTree tree, int val) {
+        tree.remove(val);
+        System.out.println("\nAfter removing node " + val + ", the AVL tree is ");
+        PrintUtil.printTree(tree.root);
+    }
+
+    public static void main(String[] args) {
+        /* Initialize empty AVL tree */
+        AVLTree avlTree = new AVLTree();
+
+        /* Insert node */
+        // Notice how the AVL tree maintains balance after inserting nodes
+        testInsert(avlTree, 1);
+        testInsert(avlTree, 2);
+        testInsert(avlTree, 3);
+        testInsert(avlTree, 4);
+        testInsert(avlTree, 5);
+        testInsert(avlTree, 8);
+        testInsert(avlTree, 7);
+        testInsert(avlTree, 9);
+        testInsert(avlTree, 10);
+        testInsert(avlTree, 6);
+
+        /* Insert duplicate node */
+        testInsert(avlTree, 7);
+
+        /* Remove node */
+        // Notice how the AVL tree maintains balance after removing nodes
+        testRemove(avlTree, 8); // Remove node with degree 0
+        testRemove(avlTree, 5); // Remove node with degree 1
+        testRemove(avlTree, 4); // Remove node with degree 2
+
+        /* Search node */
+        TreeNode node = avlTree.search(7);
+        System.out.println("\nThe found node object is " + node + ", node value = " + node.val);
+    }
+}

en/codes/java/chapter_tree/binary_search_tree.java

@@ -0,0 +1,158 @@
+/**
+ * File: binary_search_tree.java
+ * Created Time: 2022-11-25
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_tree;
+
+import utils.*;
+
+/* Binary search tree */
+class BinarySearchTree {
+    private TreeNode root;
+
+    /* Constructor */
+    public BinarySearchTree() {
+        // Initialize empty tree
+        root = null;
+    }
+
+    /* Get binary tree root node */
+    public TreeNode getRoot() {
+        return root;
+    }
+
+    /* Search node */
+    public TreeNode search(int num) {
+        TreeNode cur = root;
+        // Loop find, break after passing leaf nodes
+        while (cur != null) {
+            // Target node is in cur's right subtree
+            if (cur.val < num)
+                cur = cur.right;
+            // Target node is in cur's left subtree
+            else if (cur.val > num)
+                cur = cur.left;
+            // Found target node, break loop
+            else
+                break;
+        }
+        // Return target node
+        return cur;
+    }
+
+    /* Insert node */
+    public void insert(int num) {
+        // If tree is empty, initialize root node
+        if (root == null) {
+            root = new TreeNode(num);
+            return;
+        }
+        TreeNode cur = root, pre = null;
+        // Loop find, break after passing leaf nodes
+        while (cur != null) {
+            // Found duplicate node, thus return
+            if (cur.val == num)
+                return;
+            pre = cur;
+            // Insertion position is in cur's right subtree
+            if (cur.val < num)
+                cur = cur.right;
+            // Insertion position is in cur's left subtree
+            else
+                cur = cur.left;
+        }
+        // Insert node
+        TreeNode node = new TreeNode(num);
+        if (pre.val < num)
+            pre.right = node;
+        else
+            pre.left = node;
+    }
+
+    /* Remove node */
+    public void remove(int num) {
+        // If tree is empty, return
+        if (root == null)
+            return;
+        TreeNode cur = root, pre = null;
+        // Loop find, break after passing leaf nodes
+        while (cur != null) {
+            // Found node to be removed, break loop
+            if (cur.val == num)
+                break;
+            pre = cur;
+            // Node to be removed is in cur's right subtree
+            if (cur.val < num)
+                cur = cur.right;
+            // Node to be removed is in cur's left subtree
+            else
+                cur = cur.left;
+        }
+        // If no node to be removed, return
+        if (cur == null)
+            return;
+        // Number of child nodes = 0 or 1
+        if (cur.left == null || cur.right == null) {
+            // When the number of child nodes = 0/1, child = null/that child node
+            TreeNode child = cur.left != null ? cur.left : cur.right;
+            // Remove node cur
+            if (cur != root) {
+                if (pre.left == cur)
+                    pre.left = child;
+                else
+                    pre.right = child;
+            } else {
+                // If the removed node is the root, reassign the root
+                root = child;
+            }
+        }
+        // Number of child nodes = 2
+        else {
+            // Get the next node in in-order traversal of cur
+            TreeNode tmp = cur.right;
+            while (tmp.left != null) {
+                tmp = tmp.left;
+            }
+            // Recursively remove node tmp
+            remove(tmp.val);
+            // Replace cur with tmp
+            cur.val = tmp.val;
+        }
+    }
+}
+
+public class binary_search_tree {
+    public static void main(String[] args) {
+        /* Initialize binary search tree */
+        BinarySearchTree bst = new BinarySearchTree();
+        // Note that different insertion orders can result in various tree structures. This particular sequence creates a perfect binary tree
+        int[] nums = { 8, 4, 12, 2, 6, 10, 14, 1, 3, 5, 7, 9, 11, 13, 15 };
+        for (int num : nums) {
+            bst.insert(num);
+        }
+        System.out.println("\nInitialized binary tree is\n");
+        PrintUtil.printTree(bst.getRoot());
+
+        /* Search node */
+        TreeNode node = bst.search(7);
+        System.out.println("\nThe found node object is " + node + ", node value = " + node.val);
+
+        /* Insert node */
+        bst.insert(16);
+        System.out.println("\nAfter inserting node 16, the binary tree is\n");
+        PrintUtil.printTree(bst.getRoot());
+
+        /* Remove node */
+        bst.remove(1);
+        System.out.println("\nAfter removing node 1, the binary tree is\n");
+        PrintUtil.printTree(bst.getRoot());
+        bst.remove(2);
+        System.out.println("\nAfter removing node 2, the binary tree is\n");
+        PrintUtil.printTree(bst.getRoot());
+        bst.remove(4);
+        System.out.println("\nAfter removing node 4, the binary tree is\n");
+        PrintUtil.printTree(bst.getRoot());
+    }
+}

en/codes/java/chapter_tree/binary_tree.java

@@ -0,0 +1,40 @@
+/**
+ * File: binary_tree.java
+ * Created Time: 2022-11-25
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_tree;
+
+import utils.*;
+
+public class binary_tree {
+    public static void main(String[] args) {
+        /* Initialize binary tree */
+        // Initialize node
+        TreeNode n1 = new TreeNode(1);
+        TreeNode n2 = new TreeNode(2);
+        TreeNode n3 = new TreeNode(3);
+        TreeNode n4 = new TreeNode(4);
+        TreeNode n5 = new TreeNode(5);
+        // Construct node references (pointers)
+        n1.left = n2;
+        n1.right = n3;
+        n2.left = n4;
+        n2.right = n5;
+        System.out.println("\nInitialize binary tree\n");
+        PrintUtil.printTree(n1);
+
+        /* Insert and remove nodes */
+        TreeNode P = new TreeNode(0);
+        // Insert node P between n1 -> n2
+        n1.left = P;
+        P.left = n2;
+        System.out.println("\nAfter inserting node P\n");
+        PrintUtil.printTree(n1);
+        // Remove node P
+        n1.left = n2;
+        System.out.println("\nAfter removing node P\n");
+        PrintUtil.printTree(n1);
+    }
+}

en/codes/java/chapter_tree/binary_tree_bfs.java

@@ -0,0 +1,42 @@
+/**
+ * File: binary_tree_bfs.java
+ * Created Time: 2022-11-25
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_tree;
+
+import utils.*;
+import java.util.*;
+
+public class binary_tree_bfs {
+    /* Level-order traversal */
+    static List<Integer> levelOrder(TreeNode root) {
+        // Initialize queue, add root node
+        Queue<TreeNode> queue = new LinkedList<>();
+        queue.add(root);
+        // Initialize a list to store the traversal sequence
+        List<Integer> list = new ArrayList<>();
+        while (!queue.isEmpty()) {
+            TreeNode node = queue.poll(); // Queue dequeues
+            list.add(node.val);           // Save node value
+            if (node.left != null)
+                queue.offer(node.left);   // Left child node enqueues
+            if (node.right != null)
+                queue.offer(node.right);  // Right child node enqueues
+        }
+        return list;
+    }
+
+    public static void main(String[] args) {
+        /* Initialize binary tree */
+        // Use a specific function to convert an array into a binary tree
+        TreeNode root = TreeNode.listToTree(Arrays.asList(1, 2, 3, 4, 5, 6, 7));
+        System.out.println("\nInitialize binary tree\n");
+        PrintUtil.printTree(root);
+
+        /* Level-order traversal */
+        List<Integer> list = levelOrder(root);
+        System.out.println("\nPrint sequence of nodes from level-order traversal = " + list);
+    }
+}

en/codes/java/chapter_tree/binary_tree_dfs.java

@@ -0,0 +1,68 @@
+/**
+ * File: binary_tree_dfs.java
+ * Created Time: 2022-11-25
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_tree;
+
+import utils.*;
+import java.util.*;
+
+public class binary_tree_dfs {
+    // Initialize the list for storing traversal sequences
+    static ArrayList<Integer> list = new ArrayList<>();
+
+    /* Pre-order traversal */
+    static void preOrder(TreeNode root) {
+        if (root == null)
+            return;
+        // Visit priority: root node -> left subtree -> right subtree
+        list.add(root.val);
+        preOrder(root.left);
+        preOrder(root.right);
+    }
+
+    /* In-order traversal */
+    static void inOrder(TreeNode root) {
+        if (root == null)
+            return;
+        // Visit priority: left subtree -> root node -> right subtree
+        inOrder(root.left);
+        list.add(root.val);
+        inOrder(root.right);
+    }
+
+    /* Post-order traversal */
+    static void postOrder(TreeNode root) {
+        if (root == null)
+            return;
+        // Visit priority: left subtree -> right subtree -> root node
+        postOrder(root.left);
+        postOrder(root.right);
+        list.add(root.val);
+    }
+
+    public static void main(String[] args) {
+        /* Initialize binary tree */
+        // Use a specific function to convert an array into a binary tree
+        TreeNode root = TreeNode.listToTree(Arrays.asList(1, 2, 3, 4, 5, 6, 7));
+        System.out.println("\nInitialize binary tree\n");
+        PrintUtil.printTree(root);
+
+        /* Pre-order traversal */
+        list.clear();
+        preOrder(root);
+        System.out.println("\nPrint sequence of nodes from pre-order traversal = " + list);
+
+        /* In-order traversal */
+        list.clear();
+        inOrder(root);
+        System.out.println("\nPrint sequence of nodes from in-order traversal = " + list);
+
+        /* Post-order traversal */
+        list.clear();
+        postOrder(root);
+        System.out.println("\nPrint sequence of nodes from post-order traversal = " + list);
+    }
+}

en/codes/java/utils/ListNode.java

@@ -0,0 +1,28 @@
+/**
+ * File: ListNode.java
+ * Created Time: 2022-11-25
+ * Author: krahets (krahets@163.com)
+ */
+
+package utils;
+
+/* Linked list node */
+public class ListNode {
+    public int val;
+    public ListNode next;
+
+    public ListNode(int x) {
+        val = x;
+    }
+
+    /* Deserialize a list into a linked list */
+    public static ListNode arrToLinkedList(int[] arr) {
+        ListNode dum = new ListNode(0);
+        ListNode head = dum;
+        for (int val : arr) {
+            head.next = new ListNode(val);
+            head = head.next;
+        }
+        return dum.next;
+    }
+}

en/codes/java/utils/PrintUtil.java

@@ -0,0 +1,116 @@
+/**
+ * File: PrintUtil.java
+ * Created Time: 2022-11-25
+ * Author: krahets (krahets@163.com)
+ */
+
+package utils;
+
+import java.util.*;
+
+class Trunk {
+    Trunk prev;
+    String str;
+
+    Trunk(Trunk prev, String str) {
+        this.prev = prev;
+        this.str = str;
+    }
+};
+
+public class PrintUtil {
+    /* Print matrix (Array) */
+    public static <T> void printMatrix(T[][] matrix) {
+        System.out.println("[");
+        for (T[] row : matrix) {
+            System.out.println("  " + row + ",");
+        }
+        System.out.println("]");
+    }
+
+    /* Print matrix (List) */
+    public static <T> void printMatrix(List<List<T>> matrix) {
+        System.out.println("[");
+        for (List<T> row : matrix) {
+            System.out.println("  " + row + ",");
+        }
+        System.out.println("]");
+    }
+
+    /* Print linked list */
+    public static void printLinkedList(ListNode head) {
+        List<String> list = new ArrayList<>();
+        while (head != null) {
+            list.add(String.valueOf(head.val));
+            head = head.next;
+        }
+        System.out.println(String.join(" -> ", list));
+    }
+
+    /* Print binary tree */
+    public static void printTree(TreeNode root) {
+        printTree(root, null, false);
+    }
+
+    /**
+     * Print binary tree
+     * This tree printer is borrowed from TECHIE DELIGHT
+     * https://www.techiedelight.com/c-program-print-binary-tree/
+     */
+    public static void printTree(TreeNode root, Trunk prev, boolean isRight) {
+        if (root == null) {
+            return;
+        }
+
+        String prev_str = "    ";
+        Trunk trunk = new Trunk(prev, prev_str);
+
+        printTree(root.right, trunk, true);
+
+        if (prev == null) {
+            trunk.str = "———";
+        } else if (isRight) {
+            trunk.str = "/———";
+            prev_str = "   |";
+        } else {
+            trunk.str = "\\———";
+            prev.str = prev_str;
+        }
+
+        showTrunks(trunk);
+        System.out.println(" " + root.val);
+
+        if (prev != null) {
+            prev.str = prev_str;
+        }
+        trunk.str = "   |";
+
+        printTree(root.left, trunk, false);
+    }
+
+    public static void showTrunks(Trunk p) {
+        if (p == null) {
+            return;
+        }
+
+        showTrunks(p.prev);
+        System.out.print(p.str);
+    }
+
+    /* Print hash table */
+    public static <K, V> void printHashMap(Map<K, V> map) {
+        for (Map.Entry<K, V> kv : map.entrySet()) {
+            System.out.println(kv.getKey() + " -> " + kv.getValue());
+        }
+    }
+
+    /* Print heap (Priority queue) */
+    public static void printHeap(Queue<Integer> queue) {
+        List<Integer> list = new ArrayList<>(queue);
+        System.out.print("Array representation of the heap:");
+        System.out.println(list);
+        System.out.println("Tree representation of the heap:");
+        TreeNode root = TreeNode.listToTree(list);
+        printTree(root);
+    }
+}

en/codes/java/utils/TreeNode.java

@@ -0,0 +1,73 @@
+/**
+ * File: TreeNode.java
+ * Created Time: 2022-11-25
+ * Author: krahets (krahets@163.com)
+ */
+
+package utils;
+
+import java.util.*;
+
+/* Binary tree node class */
+public class TreeNode {
+    public int val; // Node value
+    public int height; // Node height
+    public TreeNode left; // Reference to the left child node
+    public TreeNode right; // Reference to the right child node
+
+    /* Constructor */
+    public TreeNode(int x) {
+        val = x;
+    }
+
+    // For serialization encoding rules, refer to:
+    // https://www.hello-algo.com/chapter_tree/array_representation_of_tree/
+    // Array representation of the binary tree:
+    // [1, 2, 3, 4, None, 6, 7, 8, 9, None, None, 12, None, None, 15]
+    // Linked list representation of the binary tree:
+    //             /——— 15
+    //         /——— 7
+    //     /——— 3
+    //    |    \——— 6
+    //    |        \——— 12
+    // ——— 1
+    //     \——— 2
+    //        |    /——— 9
+    //         \——— 4
+    //             \——— 8
+
+    /* Deserialize a list into a binary tree: Recursively */
+    private static TreeNode listToTreeDFS(List<Integer> arr, int i) {
+        if (i < 0 || i >= arr.size() || arr.get(i) == null) {
+            return null;
+        }
+        TreeNode root = new TreeNode(arr.get(i));
+        root.left = listToTreeDFS(arr, 2 * i + 1);
+        root.right = listToTreeDFS(arr, 2 * i + 2);
+        return root;
+    }
+
+    /* Deserialize a list into a binary tree */
+    public static TreeNode listToTree(List<Integer> arr) {
+        return listToTreeDFS(arr, 0);
+    }
+
+    /* Serialize a binary tree into a list: Recursively */
+    private static void treeToListDFS(TreeNode root, int i, List<Integer> res) {
+        if (root == null)
+            return;
+        while (i >= res.size()) {
+            res.add(null);
+        }
+        res.set(i, root.val);
+        treeToListDFS(root.left, 2 * i + 1, res);
+        treeToListDFS(root.right, 2 * i + 2, res);
+    }
+
+    /* Serialize a binary tree into a list */
+    public static List<Integer> treeToList(TreeNode root) {
+        List<Integer> res = new ArrayList<>();
+        treeToListDFS(root, 0, res);
+        return res;
+    }
+}

en/codes/java/utils/Vertex.java

@@ -0,0 +1,36 @@
+/**
+ * File: Vertex.java
+ * Created Time: 2023-02-15
+ * Author: krahets (krahets@163.com)
+ */
+
+package utils;
+
+import java.util.*;
+
+/* Vertex class */
+public class Vertex {
+    public int val;
+
+    public Vertex(int val) {
+        this.val = val;
+    }
+
+    /* Input a list of values vals, return a list of vertices vets */
+    public static Vertex[] valsToVets(int[] vals) {
+        Vertex[] vets = new Vertex[vals.length];
+        for (int i = 0; i < vals.length; i++) {
+            vets[i] = new Vertex(vals[i]);
+        }
+        return vets;
+    }
+
+    /* Input a list of vertices vets, return a list of values vals */
+    public static List<Integer> vetsToVals(List<Vertex> vets) {
+        List<Integer> vals = new ArrayList<>();
+        for (Vertex vet : vets) {
+            vals.add(vet.val);
+        }
+        return vals;
+    }
+}

en/codes/python/.gitignore

@@ -0,0 +1 @@
+__pycache__

en/codes/python/chapter_array_and_linkedlist/array.py

@@ -0,0 +1,100 @@
+"""
+File: array.py
+Created Time: 2022-11-25
+Author: krahets (krahets@163.com)
+"""
+
+import random
+
+
+def random_access(nums: list[int]) -> int:
+    """Random access to elements"""
+    # Randomly select a number from the interval [0, len(nums)-1]
+    random_index = random.randint(0, len(nums) - 1)
+    # Retrieve and return a random element
+    random_num = nums[random_index]
+    return random_num
+
+
+# Note that Python's list is a dynamic array that can be extended
+# For ease of learning, this function treats the list as a static array
+def extend(nums: list[int], enlarge: int) -> list[int]:
+    """Extend array length"""
+    # Initialize an extended length array
+    res = [0] * (len(nums) + enlarge)
+    # Copy all elements from the original array to the new array
+    for i in range(len(nums)):
+        res[i] = nums[i]
+    # Return the new array after expansion
+    return res
+
+
+def insert(nums: list[int], num: int, index: int):
+    """Insert element num at `index`"""
+    # Move all elements after `index` one position backward
+    for i in range(len(nums) - 1, index, -1):
+        nums[i] = nums[i - 1]
+    # Assign num to the element at index
+    nums[index] = num
+
+
+def remove(nums: list[int], index: int):
+    """Remove the element at `index`"""
+    # Move all elements after `index` one position forward
+    for i in range(index, len(nums) - 1):
+        nums[i] = nums[i + 1]
+
+
+def traverse(nums: list[int]):
+    """Traverse array"""
+    count = 0
+    # Traverse array by index
+    for i in range(len(nums)):
+        count += nums[i]
+    # Traverse array elements
+    for num in nums:
+        count += num
+    # Traverse both data index and elements
+    for i, num in enumerate(nums):
+        count += nums[i]
+        count += num
+
+
+def find(nums: list[int], target: int) -> int:
+    """Search for a specified element in the array"""
+    for i in range(len(nums)):
+        if nums[i] == target:
+            return i
+    return -1
+
+
+"""Driver Code"""
+if __name__ == "__main__":
+    # Initialize an array
+    arr = [0] * 5
+    print("Array arr =", arr)
+    nums = [1, 3, 2, 5, 4]
+    print("Array nums =", nums)
+
+    # Random access
+    random_num: int = random_access(nums)
+    print("Retrieve a random element in nums", random_num)
+
+    # Length extension
+    nums: list[int] = extend(nums, 3)
+    print("Extend the array length to 8, resulting in nums =", nums)
+
+    # Insert element
+    insert(nums, 6, 3)
+    print("Insert number 6 at index 3, resulting in nums =", nums)
+
+    # Remove element
+    remove(nums, 2)
+    print("Remove the element at index 2, resulting in nums =", nums)
+
+    # Traverse array
+    traverse(nums)
+
+    # Search for elements
+    index: int = find(nums, 3)
+    print("Search for element 3 in nums, resulting in index =", index)

en/codes/python/chapter_array_and_linkedlist/linked_list.py

@@ -0,0 +1,85 @@
+"""
+File: linked_list.py
+Created Time: 2022-11-25
+Author: krahets (krahets@163.com)
+"""
+
+import sys
+from pathlib import Path
+
+sys.path.append(str(Path(__file__).parent.parent))
+from modules import ListNode, print_linked_list
+
+
+def insert(n0: ListNode, P: ListNode):
+    """Insert node P after node n0 in the linked list"""
+    n1 = n0.next
+    P.next = n1
+    n0.next = P
+
+
+def remove(n0: ListNode):
+    """Remove the first node after node n0 in the linked list"""
+    if not n0.next:
+        return
+    # n0 -> P -> n1
+    P = n0.next
+    n1 = P.next
+    n0.next = n1
+
+
+def access(head: ListNode, index: int) -> ListNode | None:
+    """Access the node at `index` in the linked list"""
+    for _ in range(index):
+        if not head:
+            return None
+        head = head.next
+    return head
+
+
+def find(head: ListNode, target: int) -> int:
+    """Search for the first node with value target in the linked list"""
+    index = 0
+    while head:
+        if head.val == target:
+            return index
+        head = head.next
+        index += 1
+    return -1
+
+
+"""Driver Code"""
+if __name__ == "__main__":
+    # Initialize linked list
+    # Initialize each node
+    n0 = ListNode(1)
+    n1 = ListNode(3)
+    n2 = ListNode(2)
+    n3 = ListNode(5)
+    n4 = ListNode(4)
+    # Build references between nodes
+    n0.next = n1
+    n1.next = n2
+    n2.next = n3
+    n3.next = n4
+    print("The initialized linked list is")
+    print_linked_list(n0)
+
+    # Insert node
+    p = ListNode(0)
+    insert(n0, p)
+    print("Linked list after inserting the node is")
+    print_linked_list(n0)
+
+    # Remove node
+    remove(n0)
+    print("Linked list after removing the node is")
+    print_linked_list(n0)
+
+    # Access node
+    node: ListNode = access(n0, 3)
+    print("The value of the node at index 3 in the linked list = {}".format(node.val))
+
+    # Search node
+    index: int = find(n0, 2)
+    print("The index of the node with value 2 in the linked list = {}".format(index))

en/codes/python/chapter_array_and_linkedlist/list.py

@@ -0,0 +1,56 @@
+"""
+File: list.py
+Created Time: 2022-11-25
+Author: krahets (krahets@163.com)
+"""
+
+"""Driver Code"""
+if __name__ == "__main__":
+    # Initialize list
+    nums: list[int] = [1, 3, 2, 5, 4]
+    print("\nList nums =", nums)
+
+    # Access element
+    x: int = nums[1]
+    print("\nAccess the element at index 1, resulting in x =", x)
+
+    # Update element
+    nums[1] = 0
+    print("\nUpdate the element at index 1 to 0, resulting in nums =", nums)
+
+    # Clear list
+    nums.clear()
+    print("\nAfter clearing the list, nums =", nums)
+
+    # Add element at the end
+    nums.append(1)
+    nums.append(3)
+    nums.append(2)
+    nums.append(5)
+    nums.append(4)
+    print("\nAfter adding the element, nums =", nums)
+
+    # Insert element in the middle
+    nums.insert(3, 6)
+    print("\nInsert number 6 at index 3, resulting in nums =", nums)
+
+    # Remove element
+    nums.pop(3)
+    print("\nRemove the element at index 3, resulting in nums =", nums)
+
+    # Traverse the list by index
+    count = 0
+    for i in range(len(nums)):
+        count += nums[i]
+    # Traverse the list elements
+    for num in nums:
+        count += num
+
+    # Concatenate two lists
+    nums1 = [6, 8, 7, 10, 9]
+    nums += nums1
+    print("\nConcatenate list nums1 to nums, resulting in nums =", nums)
+
+    # Sort list
+    nums.sort()
+    print("\nAfter sorting the list, nums =", nums)

en/codes/python/chapter_array_and_linkedlist/my_list.py

@@ -0,0 +1,118 @@
+"""
+File: my_list.py
+Created Time: 2022-11-25
+Author: krahets (krahets@163.com)
+"""
+
+
+class MyList:
+    """List class"""
+
+    def __init__(self):
+        """Constructor"""
+        self._capacity: int = 10  # List capacity
+        self._arr: list[int] = [0] * self._capacity  # Array (stores list elements)
+        self._size: int = 0  # List length (current number of elements)
+        self._extend_ratio: int = 2  # Multiple for each list expansion
+
+    def size(self) -> int:
+        """Get list length (current number of elements)"""
+        return self._size
+
+    def capacity(self) -> int:
+        """Get list capacity"""
+        return self._capacity
+
+    def get(self, index: int) -> int:
+        """Access element"""
+        # If the index is out of bounds, throw an exception, as below
+        if index < 0 or index >= self._size:
+            raise IndexError("Index out of bounds")
+        return self._arr[index]
+
+    def set(self, num: int, index: int):
+        """Update element"""
+        if index < 0 or index >= self._size:
+            raise IndexError("Index out of bounds")
+        self._arr[index] = num
+
+    def add(self, num: int):
+        """Add element at the end"""
+        # When the number of elements exceeds capacity, trigger the expansion mechanism
+        if self.size() == self.capacity():
+            self.extend_capacity()
+        self._arr[self._size] = num
+        self._size += 1
+
+    def insert(self, num: int, index: int):
+        """Insert element in the middle"""
+        if index < 0 or index >= self._size:
+            raise IndexError("Index out of bounds")
+        # When the number of elements exceeds capacity, trigger the expansion mechanism
+        if self._size == self.capacity():
+            self.extend_capacity()
+        # Move all elements after `index` one position backward
+        for j in range(self._size - 1, index - 1, -1):
+            self._arr[j + 1] = self._arr[j]
+        self._arr[index] = num
+        # Update the number of elements
+        self._size += 1
+
+    def remove(self, index: int) -> int:
+        """Remove element"""
+        if index < 0 or index >= self._size:
+            raise IndexError("Index out of bounds")
+        num = self._arr[index]
+        # Move all elements after `index` one position forward
+        for j in range(index, self._size - 1):
+            self._arr[j] = self._arr[j + 1]
+        # Update the number of elements
+        self._size -= 1
+        # Return the removed element
+        return num
+
+    def extend_capacity(self):
+        """Extend list"""
+        # Create a new array of _extend_ratio times the length of the original array and copy the original array to the new array
+        self._arr = self._arr + [0] * self.capacity() * (self._extend_ratio - 1)
+        # Update list capacity
+        self._capacity = len(self._arr)
+
+    def to_array(self) -> list[int]:
+        """Return a list of valid lengths"""
+        return self._arr[: self._size]
+
+
+"""Driver Code"""
+if __name__ == "__main__":
+    # Initialize list
+    nums = MyList()
+    # Add element at the end
+    nums.add(1)
+    nums.add(3)
+    nums.add(2)
+    nums.add(5)
+    nums.add(4)
+    print(f"List nums = {nums.to_array()} ，capacity = {nums.capacity()} ，length = {nums.size()}")
+
+    # Insert element in the middle
+    nums.insert(6, index=3)
+    print("Insert number 6 at index 3, resulting in nums =", nums.to_array())
+
+    # Remove element
+    nums.remove(3)
+    print("Remove the element at index 3, resulting in nums =", nums.to_array())
+
+    # Access element
+    num = nums.get(1)
+    print("Access the element at index 1, resulting in num =", num)
+
+    # Update element
+    nums.set(0, 1)
+    print("Update the element at index 1 to 0, resulting in nums =", nums.to_array())
+
+    # Test expansion mechanism
+    for i in range(10):
+        # At i = 5, the list length will exceed the list capacity, triggering the expansion mechanism at this time
+        nums.add(i)
+    print(f"After expansion, the list {nums.to_array()} ，capacity = {nums.capacity()} ，length = {nums.size()}")

en/codes/python/chapter_backtracking/n_queens.py

@@ -0,0 +1,62 @@
+"""
+File: n_queens.py
+Created Time: 2023-04-26
+Author: krahets (krahets@163.com)
+"""
+
+
+def backtrack(
+    row: int,
+    n: int,
+    state: list[list[str]],
+    res: list[list[list[str]]],
+    cols: list[bool],
+    diags1: list[bool],
+    diags2: list[bool],
+):
+    """Backtracking algorithm: n queens"""
+    # When all rows are placed, record the solution
+    if row == n:
+        res.append([list(row) for row in state])
+        return
+    # Traverse all columns
+    for col in range(n):
+        # Calculate the main and minor diagonals corresponding to the cell
+        diag1 = row - col + n - 1
+        diag2 = row + col
+        # Pruning: do not allow queens on the column, main diagonal, or minor diagonal of the cell
+        if not cols[col] and not diags1[diag1] and not diags2[diag2]:
+            # Attempt: place the queen in the cell
+            state[row][col] = "Q"
+            cols[col] = diags1[diag1] = diags2[diag2] = True
+            # Place the next row
+            backtrack(row + 1, n, state, res, cols, diags1, diags2)
+            # Retract: restore the cell to an empty spot
+            state[row][col] = "#"
+            cols[col] = diags1[diag1] = diags2[diag2] = False
+
+
+def n_queens(n: int) -> list[list[list[str]]]:
+    """Solve n queens"""
+    # Initialize an n*n size chessboard, where 'Q' represents the queen and '#' represents an empty spot
+    state = [["#" for _ in range(n)] for _ in range(n)]
+    cols = [False] * n  # Record columns with queens
+    diags1 = [False] * (2 * n - 1)  # Record main diagonals with queens
+    diags2 = [False] * (2 * n - 1)  # Record minor diagonals with queens
+    res = []
+    backtrack(0, n, state, res, cols, diags1, diags2)
+
+    return res
+
+
+"""Driver Code"""
+if __name__ == "__main__":
+    n = 4
+    res = n_queens(n)
+
+    print(f"Input chessboard dimensions as {n}")
+    print(f"The total number of queen placement solutions is {len(res)}")
+    for state in res:
+        print("--------------------")
+        for row in state:
+            print(row)

en/codes/python/chapter_backtracking/permutations_i.py

@@ -0,0 +1,44 @@
+"""
+File: permutations_i.py
+Created Time: 2023-04-15
+Author: krahets (krahets@163.com)
+"""
+
+
+def backtrack(
+    state: list[int], choices: list[int], selected: list[bool], res: list[list[int]]
+):
+    """Backtracking algorithm: Permutation I"""
+    # When the state length equals the number of elements, record the solution
+    if len(state) == len(choices):
+        res.append(list(state))
+        return
+    # Traverse all choices
+    for i, choice in enumerate(choices):
+        # Pruning: do not allow repeated selection of elements
+        if not selected[i]:
+            # Attempt: make a choice, update the state
+            selected[i] = True
+            state.append(choice)
+            # Proceed to the next round of selection
+            backtrack(state, choices, selected, res)
+            # Retract: undo the choice, restore to the previous state
+            selected[i] = False
+            state.pop()
+
+
+def permutations_i(nums: list[int]) -> list[list[int]]:
+    """Permutation I"""
+    res = []
+    backtrack(state=[], choices=nums, selected=[False] * len(nums), res=res)
+    return res
+
+
+"""Driver Code"""
+if __name__ == "__main__":
+    nums = [1, 2, 3]
+
+    res = permutations_i(nums)
+
+    print(f"Input array nums = {nums}")
+    print(f"All permutations res = {res}")

en/codes/python/chapter_backtracking/permutations_ii.py

@@ -0,0 +1,46 @@
+"""
+File: permutations_ii.py
+Created Time: 2023-04-15
+Author: krahets (krahets@163.com)
+"""
+
+
+def backtrack(
+    state: list[int], choices: list[int], selected: list[bool], res: list[list[int]]
+):
+    """Backtracking algorithm: Permutation II"""
+    # When the state length equals the number of elements, record the solution
+    if len(state) == len(choices):
+        res.append(list(state))
+        return
+    # Traverse all choices
+    duplicated = set[int]()
+    for i, choice in enumerate(choices):
+        # Pruning: do not allow repeated selection of elements and do not allow repeated selection of equal elements
+        if not selected[i] and choice not in duplicated:
+            # Attempt: make a choice, update the state
+            duplicated.add(choice)  # Record selected element values
+            selected[i] = True
+            state.append(choice)
+            # Proceed to the next round of selection
+            backtrack(state, choices, selected, res)
+            # Retract: undo the choice, restore to the previous state
+            selected[i] = False
+            state.pop()
+
+
+def permutations_ii(nums: list[int]) -> list[list[int]]:
+    """Permutation II"""
+    res = []
+    backtrack(state=[], choices=nums, selected=[False] * len(nums), res=res)
+    return res
+
+
+"""Driver Code"""
+if __name__ == "__main__":
+    nums = [1, 2, 2]
+
+    res = permutations_ii(nums)
+
+    print(f"Input array nums = {nums}")
+    print(f"All permutations res = {res}")

en/codes/python/chapter_backtracking/preorder_traversal_i_compact.py

@@ -0,0 +1,36 @@
+"""
+File: preorder_traversal_i_compact.py
+Created Time: 2023-04-15
+Author: krahets (krahets@163.com)
+"""
+
+import sys
+from pathlib import Path
+
+sys.path.append(str(Path(__file__).parent.parent))
+from modules import TreeNode, print_tree, list_to_tree
+
+
+def pre_order(root: TreeNode):
+    """Pre-order traversal: Example one"""
+    if root is None:
+        return
+    if root.val == 7:
+        # Record solution
+        res.append(root)
+    pre_order(root.left)
+    pre_order(root.right)
+
+
+"""Driver Code"""
+if __name__ == "__main__":
+    root = list_to_tree([1, 7, 3, 4, 5, 6, 7])
+    print("\nInitialize binary tree")
+    print_tree(root)
+
+    # Pre-order traversal
+    res = list[TreeNode]()
+    pre_order(root)
+
+    print("\nOutput all nodes with value 7")
+    print([node.val for node in res])

en/codes/python/chapter_backtracking/preorder_traversal_ii_compact.py

@@ -0,0 +1,42 @@
+"""
+File: preorder_traversal_ii_compact.py
+Created Time: 2023-04-15
+Author: krahets (krahets@163.com)
+"""
+
+import sys
+from pathlib import Path
+
+sys.path.append(str(Path(__file__).parent.parent))
+from modules import TreeNode, print_tree, list_to_tree
+
+
+def pre_order(root: TreeNode):
+    """Pre-order traversal: Example two"""
+    if root is None:
+        return
+    # Attempt
+    path.append(root)
+    if root.val == 7:
+        # Record solution
+        res.append(list(path))
+    pre_order(root.left)
+    pre_order(root.right)
+    # Retract
+    path.pop()
+
+
+"""Driver Code"""
+if __name__ == "__main__":
+    root = list_to_tree([1, 7, 3, 4, 5, 6, 7])
+    print("\nInitialize binary tree")
+    print_tree(root)
+
+    # Pre-order traversal
+    path = list[TreeNode]()
+    res = list[list[TreeNode]]()
+    pre_order(root)
+
+    print("\nOutput all root-to-node 7 paths")
+    for path in res:
+        print([node.val for node in path])

en/codes/python/chapter_backtracking/preorder_traversal_iii_compact.py

@@ -0,0 +1,43 @@
+"""
+File: preorder_traversal_iii_compact.py
+Created Time: 2023-04-15
+Author: krahets (krahets@163.com)
+"""
+
+import sys
+from pathlib import Path
+
+sys.path.append(str(Path(__file__).parent.parent))
+from modules import TreeNode, print_tree, list_to_tree
+
+
+def pre_order(root: TreeNode):
+    """Pre-order traversal: Example three"""
+    # Pruning
+    if root is None or root.val == 3:
+        return
+    # Attempt
+    path.append(root)
+    if root.val == 7:
+        # Record solution
+        res.append(list(path))
+    pre_order(root.left)
+    pre_order(root.right)
+    # Retract
+    path.pop()
+
+
+"""Driver Code"""
+if __name__ == "__main__":
+    root = list_to_tree([1, 7, 3, 4, 5, 6, 7])
+    print("\nInitialize binary tree")
+    print_tree(root)
+
+    # Pre-order traversal
+    path = list[TreeNode]()
+    res = list[list[TreeNode]]()
+    pre_order(root)
+
+    print("\nOutput all root-to-node 7 paths, not including nodes with value 3")
+    for path in res:
+        print([node.val for node in path])

en/codes/python/chapter_backtracking/preorder_traversal_iii_template.py

@@ -0,0 +1,71 @@
+"""
+File: preorder_traversal_iii_template.py
+Created Time: 2023-04-15
+Author: krahets (krahets@163.com)
+"""
+
+import sys
+from pathlib import Path
+
+sys.path.append(str(Path(__file__).parent.parent))
+from modules import TreeNode, print_tree, list_to_tree
+
+
+def is_solution(state: list[TreeNode]) -> bool:
+    """Determine if the current state is a solution"""
+    return state and state[-1].val == 7
+
+
+def record_solution(state: list[TreeNode], res: list[list[TreeNode]]):
+    """Record solution"""
+    res.append(list(state))
+
+
+def is_valid(state: list[TreeNode], choice: TreeNode) -> bool:
+    """Determine if the choice is legal under the current state"""
+    return choice is not None and choice.val != 3
+
+
+def make_choice(state: list[TreeNode], choice: TreeNode):
+    """Update state"""
+    state.append(choice)
+
+
+def undo_choice(state: list[TreeNode], choice: TreeNode):
+    """Restore state"""
+    state.pop()
+
+
+def backtrack(
+    state: list[TreeNode], choices: list[TreeNode], res: list[list[TreeNode]]
+):
+    """Backtracking algorithm: Example three"""
+    # Check if it's a solution
+    if is_solution(state):
+        # Record solution
+        record_solution(state, res)
+    # Traverse all choices
+    for choice in choices:
+        # Pruning: check if the choice is legal
+        if is_valid(state, choice):
+            # Attempt: make a choice, update the state
+            make_choice(state, choice)
+            # Proceed to the next round of selection
+            backtrack(state, [choice.left, choice.right], res)
+            # Retract: undo the choice, restore to the previous state
+            undo_choice(state, choice)
+
+
+"""Driver Code"""
+if __name__ == "__main__":
+    root = list_to_tree([1, 7, 3, 4, 5, 6, 7])
+    print("\nInitialize binary tree")
+    print_tree(root)
+
+    # Backtracking algorithm
+    res = []
+    backtrack(state=[], choices=[root], res=res)
+
+    print("\nOutput all root-to-node 7 paths, requiring paths not to include nodes with value 3")
+    for path in res:
+        print([node.val for node in path])

en/codes/python/chapter_backtracking/subset_sum_i.py

@@ -0,0 +1,48 @@
+"""
+File: subset_sum_i.py
+Created Time: 2023-06-17
+Author: krahets (krahets@163.com)
+"""
+
+
+def backtrack(
+    state: list[int], target: int, choices: list[int], start: int, res: list[list[int]]
+):
+    """Backtracking algorithm: Subset Sum I"""
+    # When the subset sum equals target, record the solution
+    if target == 0:
+        res.append(list(state))
+        return
+    # Traverse all choices
+    # Pruning two: start traversing from start to avoid generating duplicate subsets
+    for i in range(start, len(choices)):
+        # Pruning one: if the subset sum exceeds target, end the loop immediately
+        # This is because the array is sorted, and later elements are larger, so the subset sum will definitely exceed target
+        if target - choices[i] < 0:
+            break
+        # Attempt: make a choice, update target, start
+        state.append(choices[i])
+        # Proceed to the next round of selection
+        backtrack(state, target - choices[i], choices, i, res)
+        # Retract: undo the choice, restore to the previous state
+        state.pop()
+
+
+def subset_sum_i(nums: list[int], target: int) -> list[list[int]]:
+    """Solve Subset Sum I"""
+    state = []  # State (subset)
+    nums.sort()  # Sort nums
+    start = 0  # Start point for traversal
+    res = []  # Result list (subset list)
+    backtrack(state, target, nums, start, res)
+    return res
+
+
+"""Driver Code"""
+if __name__ == "__main__":
+    nums = [3, 4, 5]
+    target = 9
+    res = subset_sum_i(nums, target)
+
+    print(f"Input array nums = {nums}, target = {target}")
+    print(f"All subsets equal to {target} res = {res}")

en/codes/python/chapter_backtracking/subset_sum_i_naive.py

@@ -0,0 +1,50 @@
+"""
+File: subset_sum_i_naive.py
+Created Time: 2023-06-17
+Author: krahets (krahets@163.com)
+"""
+
+
+def backtrack(
+    state: list[int],
+    target: int,
+    total: int,
+    choices: list[int],
+    res: list[list[int]],
+):
+    """Backtracking algorithm: Subset Sum I"""
+    # When the subset sum equals target, record the solution
+    if total == target:
+        res.append(list(state))
+        return
+    # Traverse all choices
+    for i in range(len(choices)):
+        # Pruning: if the subset sum exceeds target, skip that choice
+        if total + choices[i] > target:
+            continue
+        # Attempt: make a choice, update elements and total
+        state.append(choices[i])
+        # Proceed to the next round of selection
+        backtrack(state, target, total + choices[i], choices, res)
+        # Retract: undo the choice, restore to the previous state
+        state.pop()
+
+
+def subset_sum_i_naive(nums: list[int], target: int) -> list[list[int]]:
+    """Solve Subset Sum I (including duplicate subsets)"""
+    state = []  # State (subset)
+    total = 0  # Subset sum
+    res = []  # Result list (subset list)
+    backtrack(state, target, total, nums, res)
+    return res
+
+
+"""Driver Code"""
+if __name__ == "__main__":
+    nums = [3, 4, 5]
+    target = 9
+    res = subset_sum_i_naive(nums, target)
+
+    print(f"Input array nums = {nums}, target = {target}")
+    print(f"All subsets equal to {target} res = {res}")
+    print(f"Please note that the result of this method includes duplicate sets")