hello-algo/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);
}
}