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662 lines
20 KiB
Markdown
662 lines
20 KiB
Markdown
# 二元樹
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<u>二元樹(binary tree)</u>是一種非線性資料結構,代表“祖先”與“後代”之間的派生關係,體現了“一分為二”的分治邏輯。與鏈結串列類似,二元樹的基本單元是節點,每個節點包含值、左子節點引用和右子節點引用。
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=== "Python"
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```python title=""
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class TreeNode:
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"""二元樹節點類別"""
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def __init__(self, val: int):
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self.val: int = val # 節點值
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self.left: TreeNode | None = None # 左子節點引用
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self.right: TreeNode | None = None # 右子節點引用
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```
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=== "C++"
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```cpp title=""
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/* 二元樹節點結構體 */
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struct TreeNode {
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int val; // 節點值
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TreeNode *left; // 左子節點指標
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TreeNode *right; // 右子節點指標
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TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
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};
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```
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=== "Java"
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```java title=""
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/* 二元樹節點類別 */
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class TreeNode {
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int val; // 節點值
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TreeNode left; // 左子節點引用
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TreeNode right; // 右子節點引用
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TreeNode(int x) { val = x; }
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}
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```
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=== "C#"
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```csharp title=""
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/* 二元樹節點類別 */
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class TreeNode(int? x) {
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public int? val = x; // 節點值
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public TreeNode? left; // 左子節點引用
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public TreeNode? right; // 右子節點引用
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}
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```
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=== "Go"
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```go title=""
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/* 二元樹節點結構體 */
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type TreeNode struct {
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Val int
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Left *TreeNode
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Right *TreeNode
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}
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/* 建構子 */
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func NewTreeNode(v int) *TreeNode {
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return &TreeNode{
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Left: nil, // 左子節點指標
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Right: nil, // 右子節點指標
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Val: v, // 節點值
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}
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}
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```
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=== "Swift"
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```swift title=""
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/* 二元樹節點類別 */
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class TreeNode {
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var val: Int // 節點值
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var left: TreeNode? // 左子節點引用
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var right: TreeNode? // 右子節點引用
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init(x: Int) {
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val = x
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}
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}
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```
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=== "JS"
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```javascript title=""
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/* 二元樹節點類別 */
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class TreeNode {
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val; // 節點值
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left; // 左子節點指標
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right; // 右子節點指標
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constructor(val, left, right) {
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this.val = val === undefined ? 0 : val;
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this.left = left === undefined ? null : left;
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this.right = right === undefined ? null : right;
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}
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}
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```
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=== "TS"
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```typescript title=""
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/* 二元樹節點類別 */
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class TreeNode {
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val: number;
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left: TreeNode | null;
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right: TreeNode | null;
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constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
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this.val = val === undefined ? 0 : val; // 節點值
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this.left = left === undefined ? null : left; // 左子節點引用
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this.right = right === undefined ? null : right; // 右子節點引用
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}
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}
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```
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=== "Dart"
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```dart title=""
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/* 二元樹節點類別 */
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class TreeNode {
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int val; // 節點值
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TreeNode? left; // 左子節點引用
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TreeNode? right; // 右子節點引用
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TreeNode(this.val, [this.left, this.right]);
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}
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```
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=== "Rust"
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```rust title=""
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use std::rc::Rc;
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use std::cell::RefCell;
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/* 二元樹節點結構體 */
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struct TreeNode {
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val: i32, // 節點值
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left: Option<Rc<RefCell<TreeNode>>>, // 左子節點引用
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right: Option<Rc<RefCell<TreeNode>>>, // 右子節點引用
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}
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impl TreeNode {
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/* 建構子 */
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fn new(val: i32) -> Rc<RefCell<Self>> {
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Rc::new(RefCell::new(Self {
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val,
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left: None,
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right: None
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}))
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}
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}
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```
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=== "C"
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```c title=""
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/* 二元樹節點結構體 */
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typedef struct TreeNode {
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int val; // 節點值
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int height; // 節點高度
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struct TreeNode *left; // 左子節點指標
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struct TreeNode *right; // 右子節點指標
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} TreeNode;
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/* 建構子 */
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TreeNode *newTreeNode(int val) {
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TreeNode *node;
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node = (TreeNode *)malloc(sizeof(TreeNode));
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node->val = val;
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node->height = 0;
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node->left = NULL;
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node->right = NULL;
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return node;
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}
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```
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=== "Kotlin"
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```kotlin title=""
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/* 二元樹節點類別 */
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class TreeNode(val _val: Int) { // 節點值
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val left: TreeNode? = null // 左子節點引用
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val right: TreeNode? = null // 右子節點引用
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}
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```
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=== "Ruby"
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```ruby title=""
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```
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=== "Zig"
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```zig title=""
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```
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每個節點都有兩個引用(指標),分別指向<u>左子節點(left-child node)</u>和<u>右子節點(right-child node)</u>,該節點被稱為這兩個子節點的<u>父節點(parent node)</u>。當給定一個二元樹的節點時,我們將該節點的左子節點及其以下節點形成的樹稱為該節點的<u>左子樹(left subtree)</u>,同理可得<u>右子樹(right subtree)</u>。
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**在二元樹中,除葉節點外,其他所有節點都包含子節點和非空子樹**。如下圖所示,如果將“節點 2”視為父節點,則其左子節點和右子節點分別是“節點 4”和“節點 5”,左子樹是“節點 4 及其以下節點形成的樹”,右子樹是“節點 5 及其以下節點形成的樹”。
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![父節點、子節點、子樹](binary_tree.assets/binary_tree_definition.png)
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## 二元樹常見術語
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二元樹的常用術語如下圖所示。
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- <u>根節點(root node)</u>:位於二元樹頂層的節點,沒有父節點。
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- <u>葉節點(leaf node)</u>:沒有子節點的節點,其兩個指標均指向 `None` 。
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- <u>邊(edge)</u>:連線兩個節點的線段,即節點引用(指標)。
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- 節點所在的<u>層(level)</u>:從頂至底遞增,根節點所在層為 1 。
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- 節點的<u>度(degree)</u>:節點的子節點的數量。在二元樹中,度的取值範圍是 0、1、2 。
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- 二元樹的<u>高度(height)</u>:從根節點到最遠葉節點所經過的邊的數量。
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- 節點的<u>深度(depth)</u>:從根節點到該節點所經過的邊的數量。
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- 節點的<u>高度(height)</u>:從距離該節點最遠的葉節點到該節點所經過的邊的數量。
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![二元樹的常用術語](binary_tree.assets/binary_tree_terminology.png)
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!!! tip
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請注意,我們通常將“高度”和“深度”定義為“經過的邊的數量”,但有些題目或教材可能會將其定義為“經過的節點的數量”。在這種情況下,高度和深度都需要加 1 。
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## 二元樹基本操作
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### 初始化二元樹
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與鏈結串列類似,首先初始化節點,然後構建引用(指標)。
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=== "Python"
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```python title="binary_tree.py"
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# 初始化二元樹
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# 初始化節點
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n1 = TreeNode(val=1)
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n2 = TreeNode(val=2)
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n3 = TreeNode(val=3)
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n4 = TreeNode(val=4)
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n5 = TreeNode(val=5)
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# 構建節點之間的引用(指標)
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n1.left = n2
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n1.right = n3
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n2.left = n4
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n2.right = n5
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```
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=== "C++"
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```cpp title="binary_tree.cpp"
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/* 初始化二元樹 */
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// 初始化節點
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TreeNode* n1 = new TreeNode(1);
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TreeNode* n2 = new TreeNode(2);
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TreeNode* n3 = new TreeNode(3);
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TreeNode* n4 = new TreeNode(4);
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TreeNode* n5 = new TreeNode(5);
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// 構建節點之間的引用(指標)
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n1->left = n2;
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n1->right = n3;
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n2->left = n4;
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n2->right = n5;
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```
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=== "Java"
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```java title="binary_tree.java"
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// 初始化節點
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TreeNode n1 = new TreeNode(1);
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TreeNode n2 = new TreeNode(2);
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TreeNode n3 = new TreeNode(3);
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TreeNode n4 = new TreeNode(4);
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TreeNode n5 = new TreeNode(5);
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// 構建節點之間的引用(指標)
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n1.left = n2;
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n1.right = n3;
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n2.left = n4;
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n2.right = n5;
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```
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=== "C#"
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```csharp title="binary_tree.cs"
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/* 初始化二元樹 */
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// 初始化節點
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TreeNode n1 = new(1);
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TreeNode n2 = new(2);
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TreeNode n3 = new(3);
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TreeNode n4 = new(4);
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TreeNode n5 = new(5);
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// 構建節點之間的引用(指標)
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n1.left = n2;
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n1.right = n3;
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n2.left = n4;
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n2.right = n5;
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```
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=== "Go"
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```go title="binary_tree.go"
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/* 初始化二元樹 */
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// 初始化節點
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n1 := NewTreeNode(1)
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n2 := NewTreeNode(2)
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n3 := NewTreeNode(3)
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n4 := NewTreeNode(4)
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n5 := NewTreeNode(5)
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// 構建節點之間的引用(指標)
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n1.Left = n2
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n1.Right = n3
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n2.Left = n4
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n2.Right = n5
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```
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=== "Swift"
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```swift title="binary_tree.swift"
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// 初始化節點
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let n1 = TreeNode(x: 1)
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let n2 = TreeNode(x: 2)
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let n3 = TreeNode(x: 3)
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let n4 = TreeNode(x: 4)
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let n5 = TreeNode(x: 5)
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// 構建節點之間的引用(指標)
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n1.left = n2
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n1.right = n3
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n2.left = n4
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n2.right = n5
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```
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=== "JS"
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```javascript title="binary_tree.js"
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/* 初始化二元樹 */
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// 初始化節點
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let n1 = new TreeNode(1),
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n2 = new TreeNode(2),
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n3 = new TreeNode(3),
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n4 = new TreeNode(4),
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n5 = new TreeNode(5);
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// 構建節點之間的引用(指標)
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n1.left = n2;
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n1.right = n3;
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n2.left = n4;
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n2.right = n5;
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```
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=== "TS"
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```typescript title="binary_tree.ts"
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/* 初始化二元樹 */
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// 初始化節點
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let n1 = new TreeNode(1),
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n2 = new TreeNode(2),
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n3 = new TreeNode(3),
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n4 = new TreeNode(4),
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n5 = new TreeNode(5);
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// 構建節點之間的引用(指標)
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n1.left = n2;
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n1.right = n3;
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n2.left = n4;
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n2.right = n5;
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```
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=== "Dart"
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```dart title="binary_tree.dart"
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/* 初始化二元樹 */
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// 初始化節點
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TreeNode n1 = new TreeNode(1);
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TreeNode n2 = new TreeNode(2);
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TreeNode n3 = new TreeNode(3);
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TreeNode n4 = new TreeNode(4);
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TreeNode n5 = new TreeNode(5);
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// 構建節點之間的引用(指標)
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n1.left = n2;
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n1.right = n3;
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n2.left = n4;
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n2.right = n5;
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```
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=== "Rust"
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```rust title="binary_tree.rs"
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// 初始化節點
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let n1 = TreeNode::new(1);
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let n2 = TreeNode::new(2);
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let n3 = TreeNode::new(3);
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let n4 = TreeNode::new(4);
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let n5 = TreeNode::new(5);
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// 構建節點之間的引用(指標)
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n1.borrow_mut().left = Some(n2.clone());
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n1.borrow_mut().right = Some(n3);
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n2.borrow_mut().left = Some(n4);
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n2.borrow_mut().right = Some(n5);
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```
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=== "C"
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```c title="binary_tree.c"
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/* 初始化二元樹 */
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// 初始化節點
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TreeNode *n1 = newTreeNode(1);
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TreeNode *n2 = newTreeNode(2);
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TreeNode *n3 = newTreeNode(3);
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TreeNode *n4 = newTreeNode(4);
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TreeNode *n5 = newTreeNode(5);
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// 構建節點之間的引用(指標)
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n1->left = n2;
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n1->right = n3;
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n2->left = n4;
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n2->right = n5;
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```
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=== "Kotlin"
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```kotlin title="binary_tree.kt"
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// 初始化節點
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val n1 = TreeNode(1)
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val n2 = TreeNode(2)
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val n3 = TreeNode(3)
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val n4 = TreeNode(4)
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val n5 = TreeNode(5)
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// 構建節點之間的引用(指標)
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n1.left = n2
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n1.right = n3
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n2.left = n4
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n2.right = n5
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```
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=== "Ruby"
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```ruby title="binary_tree.rb"
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```
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=== "Zig"
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```zig title="binary_tree.zig"
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```
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??? pythontutor "視覺化執行"
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https://pythontutor.com/render.html#code=class%20TreeNode%3A%0A%20%20%20%20%22%22%22%E4%BA%8C%E5%85%83%E6%A8%B9%E7%AF%80%E9%BB%9E%E9%A1%9E%E5%88%A5%22%22%22%0A%20%20%20%20def%20__init__%28self%2C%20val%3A%20int%29%3A%0A%20%20%20%20%20%20%20%20self.val%3A%20int%20%3D%20val%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%23%20%E7%AF%80%E9%BB%9E%E5%80%BC%0A%20%20%20%20%20%20%20%20self.left%3A%20TreeNode%20%7C%20None%20%3D%20None%20%20%23%20%E5%B7%A6%E5%AD%90%E7%AF%80%E9%BB%9E%E5%BC%95%E7%94%A8%0A%20%20%20%20%20%20%20%20self.right%3A%20TreeNode%20%7C%20None%20%3D%20None%20%23%20%E5%8F%B3%E5%AD%90%E7%AF%80%E9%BB%9E%E5%BC%95%E7%94%A8%0A%0A%22%22%22Driver%20Code%22%22%22%0Aif%20__name__%20%3D%3D%20%22__main__%22%3A%0A%20%20%20%20%23%20%E5%88%9D%E5%A7%8B%E5%8C%96%E4%BA%8C%E5%85%83%E6%A8%B9%0A%20%20%20%20%23%20%E5%88%9D%E5%A7%8B%E5%8C%96%E7%AF%80%E9%BB%9E%0A%20%20%20%20n1%20%3D%20TreeNode%28val%3D1%29%0A%20%20%20%20n2%20%3D%20TreeNode%28val%3D2%29%0A%20%20%20%20n3%20%3D%20TreeNode%28val%3D3%29%0A%20%20%20%20n4%20%3D%20TreeNode%28val%3D4%29%0A%20%20%20%20n5%20%3D%20TreeNode%28val%3D5%29%0A%20%20%20%20%23%20%E6%A7%8B%E5%BB%BA%E7%AF%80%E9%BB%9E%E4%B9%8B%E9%96%93%E7%9A%84%E5%BC%95%E7%94%A8%EF%BC%88%E6%8C%87%E6%A8%99%EF%BC%89%0A%20%20%20%20n1.left%20%3D%20n2%0A%20%20%20%20n1.right%20%3D%20n3%0A%20%20%20%20n2.left%20%3D%20n4%0A%20%20%20%20n2.right%20%3D%20n5&cumulative=false&curInstr=3&heapPrimitives=nevernest&mode=display&origin=opt-frontend.js&py=311&rawInputLstJSON=%5B%5D&textReferences=false
|
||
|
||
### 插入與刪除節點
|
||
|
||
與鏈結串列類似,在二元樹中插入與刪除節點可以透過修改指標來實現。下圖給出了一個示例。
|
||
|
||
![在二元樹中插入與刪除節點](binary_tree.assets/binary_tree_add_remove.png)
|
||
|
||
=== "Python"
|
||
|
||
```python title="binary_tree.py"
|
||
# 插入與刪除節點
|
||
p = TreeNode(0)
|
||
# 在 n1 -> n2 中間插入節點 P
|
||
n1.left = p
|
||
p.left = n2
|
||
# 刪除節點 P
|
||
n1.left = n2
|
||
```
|
||
|
||
=== "C++"
|
||
|
||
```cpp title="binary_tree.cpp"
|
||
/* 插入與刪除節點 */
|
||
TreeNode* P = new TreeNode(0);
|
||
// 在 n1 -> n2 中間插入節點 P
|
||
n1->left = P;
|
||
P->left = n2;
|
||
// 刪除節點 P
|
||
n1->left = n2;
|
||
```
|
||
|
||
=== "Java"
|
||
|
||
```java title="binary_tree.java"
|
||
TreeNode P = new TreeNode(0);
|
||
// 在 n1 -> n2 中間插入節點 P
|
||
n1.left = P;
|
||
P.left = n2;
|
||
// 刪除節點 P
|
||
n1.left = n2;
|
||
```
|
||
|
||
=== "C#"
|
||
|
||
```csharp title="binary_tree.cs"
|
||
/* 插入與刪除節點 */
|
||
TreeNode P = new(0);
|
||
// 在 n1 -> n2 中間插入節點 P
|
||
n1.left = P;
|
||
P.left = n2;
|
||
// 刪除節點 P
|
||
n1.left = n2;
|
||
```
|
||
|
||
=== "Go"
|
||
|
||
```go title="binary_tree.go"
|
||
/* 插入與刪除節點 */
|
||
// 在 n1 -> n2 中間插入節點 P
|
||
p := NewTreeNode(0)
|
||
n1.Left = p
|
||
p.Left = n2
|
||
// 刪除節點 P
|
||
n1.Left = n2
|
||
```
|
||
|
||
=== "Swift"
|
||
|
||
```swift title="binary_tree.swift"
|
||
let P = TreeNode(x: 0)
|
||
// 在 n1 -> n2 中間插入節點 P
|
||
n1.left = P
|
||
P.left = n2
|
||
// 刪除節點 P
|
||
n1.left = n2
|
||
```
|
||
|
||
=== "JS"
|
||
|
||
```javascript title="binary_tree.js"
|
||
/* 插入與刪除節點 */
|
||
let P = new TreeNode(0);
|
||
// 在 n1 -> n2 中間插入節點 P
|
||
n1.left = P;
|
||
P.left = n2;
|
||
// 刪除節點 P
|
||
n1.left = n2;
|
||
```
|
||
|
||
=== "TS"
|
||
|
||
```typescript title="binary_tree.ts"
|
||
/* 插入與刪除節點 */
|
||
const P = new TreeNode(0);
|
||
// 在 n1 -> n2 中間插入節點 P
|
||
n1.left = P;
|
||
P.left = n2;
|
||
// 刪除節點 P
|
||
n1.left = n2;
|
||
```
|
||
|
||
=== "Dart"
|
||
|
||
```dart title="binary_tree.dart"
|
||
/* 插入與刪除節點 */
|
||
TreeNode P = new TreeNode(0);
|
||
// 在 n1 -> n2 中間插入節點 P
|
||
n1.left = P;
|
||
P.left = n2;
|
||
// 刪除節點 P
|
||
n1.left = n2;
|
||
```
|
||
|
||
=== "Rust"
|
||
|
||
```rust title="binary_tree.rs"
|
||
let p = TreeNode::new(0);
|
||
// 在 n1 -> n2 中間插入節點 P
|
||
n1.borrow_mut().left = Some(p.clone());
|
||
p.borrow_mut().left = Some(n2.clone());
|
||
// 刪除節點 p
|
||
n1.borrow_mut().left = Some(n2);
|
||
```
|
||
|
||
=== "C"
|
||
|
||
```c title="binary_tree.c"
|
||
/* 插入與刪除節點 */
|
||
TreeNode *P = newTreeNode(0);
|
||
// 在 n1 -> n2 中間插入節點 P
|
||
n1->left = P;
|
||
P->left = n2;
|
||
// 刪除節點 P
|
||
n1->left = n2;
|
||
```
|
||
|
||
=== "Kotlin"
|
||
|
||
```kotlin title="binary_tree.kt"
|
||
val P = TreeNode(0)
|
||
// 在 n1 -> n2 中間插入節點 P
|
||
n1.left = P
|
||
P.left = n2
|
||
// 刪除節點 P
|
||
n1.left = n2
|
||
```
|
||
|
||
=== "Ruby"
|
||
|
||
```ruby title="binary_tree.rb"
|
||
|
||
```
|
||
|
||
=== "Zig"
|
||
|
||
```zig title="binary_tree.zig"
|
||
|
||
```
|
||
|
||
??? pythontutor "視覺化執行"
|
||
|
||
https://pythontutor.com/render.html#code=class%20TreeNode%3A%0A%20%20%20%20%22%22%22%E4%BA%8C%E5%85%83%E6%A8%B9%E7%AF%80%E9%BB%9E%E9%A1%9E%E5%88%A5%22%22%22%0A%20%20%20%20def%20__init__%28self%2C%20val%3A%20int%29%3A%0A%20%20%20%20%20%20%20%20self.val%3A%20int%20%3D%20val%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%23%20%E7%AF%80%E9%BB%9E%E5%80%BC%0A%20%20%20%20%20%20%20%20self.left%3A%20TreeNode%20%7C%20None%20%3D%20None%20%20%23%20%E5%B7%A6%E5%AD%90%E7%AF%80%E9%BB%9E%E5%BC%95%E7%94%A8%0A%20%20%20%20%20%20%20%20self.right%3A%20TreeNode%20%7C%20None%20%3D%20None%20%23%20%E5%8F%B3%E5%AD%90%E7%AF%80%E9%BB%9E%E5%BC%95%E7%94%A8%0A%0A%22%22%22Driver%20Code%22%22%22%0Aif%20__name__%20%3D%3D%20%22__main__%22%3A%0A%20%20%20%20%23%20%E5%88%9D%E5%A7%8B%E5%8C%96%E4%BA%8C%E5%85%83%E6%A8%B9%0A%20%20%20%20%23%20%E5%88%9D%E5%A7%8B%E5%8C%96%E7%AF%80%E9%BB%9E%0A%20%20%20%20n1%20%3D%20TreeNode%28val%3D1%29%0A%20%20%20%20n2%20%3D%20TreeNode%28val%3D2%29%0A%20%20%20%20n3%20%3D%20TreeNode%28val%3D3%29%0A%20%20%20%20n4%20%3D%20TreeNode%28val%3D4%29%0A%20%20%20%20n5%20%3D%20TreeNode%28val%3D5%29%0A%20%20%20%20%23%20%E6%A7%8B%E5%BB%BA%E7%AF%80%E9%BB%9E%E4%B9%8B%E9%96%93%E7%9A%84%E5%BC%95%E7%94%A8%EF%BC%88%E6%8C%87%E6%A8%99%EF%BC%89%0A%20%20%20%20n1.left%20%3D%20n2%0A%20%20%20%20n1.right%20%3D%20n3%0A%20%20%20%20n2.left%20%3D%20n4%0A%20%20%20%20n2.right%20%3D%20n5%0A%0A%20%20%20%20%23%20%E6%8F%92%E5%85%A5%E8%88%87%E5%88%AA%E9%99%A4%E7%AF%80%E9%BB%9E%0A%20%20%20%20p%20%3D%20TreeNode%280%29%0A%20%20%20%20%23%20%E5%9C%A8%20n1%20-%3E%20n2%20%E4%B8%AD%E9%96%93%E6%8F%92%E5%85%A5%E7%AF%80%E9%BB%9E%20P%0A%20%20%20%20n1.left%20%3D%20p%0A%20%20%20%20p.left%20%3D%20n2%0A%20%20%20%20%23%20%E5%88%AA%E9%99%A4%E7%AF%80%E9%BB%9E%20P%0A%20%20%20%20n1.left%20%3D%20n2&cumulative=false&curInstr=37&heapPrimitives=nevernest&mode=display&origin=opt-frontend.js&py=311&rawInputLstJSON=%5B%5D&textReferences=false
|
||
|
||
!!! note
|
||
|
||
需要注意的是,插入節點可能會改變二元樹的原有邏輯結構,而刪除節點通常意味著刪除該節點及其所有子樹。因此,在二元樹中,插入與刪除通常是由一套操作配合完成的,以實現有實際意義的操作。
|
||
|
||
## 常見二元樹型別
|
||
|
||
### 完美二元樹
|
||
|
||
如下圖所示,<u>完美二元樹(perfect binary tree)</u>所有層的節點都被完全填滿。在完美二元樹中,葉節點的度為 $0$ ,其餘所有節點的度都為 $2$ ;若樹的高度為 $h$ ,則節點總數為 $2^{h+1} - 1$ ,呈現標準的指數級關係,反映了自然界中常見的細胞分裂現象。
|
||
|
||
!!! tip
|
||
|
||
請注意,在中文社群中,完美二元樹常被稱為<u>滿二元樹</u>。
|
||
|
||
![完美二元樹](binary_tree.assets/perfect_binary_tree.png)
|
||
|
||
### 完全二元樹
|
||
|
||
如下圖所示,<u>完全二元樹(complete binary tree)</u>只有最底層的節點未被填滿,且最底層節點儘量靠左填充。
|
||
|
||
![完全二元樹](binary_tree.assets/complete_binary_tree.png)
|
||
|
||
### 完滿二元樹
|
||
|
||
如下圖所示,<u>完滿二元樹(full binary tree)</u>除了葉節點之外,其餘所有節點都有兩個子節點。
|
||
|
||
![完滿二元樹](binary_tree.assets/full_binary_tree.png)
|
||
|
||
### 平衡二元樹
|
||
|
||
如下圖所示,<u>平衡二元樹(balanced binary tree)</u>中任意節點的左子樹和右子樹的高度之差的絕對值不超過 1 。
|
||
|
||
![平衡二元樹](binary_tree.assets/balanced_binary_tree.png)
|
||
|
||
## 二元樹的退化
|
||
|
||
下圖展示了二元樹的理想結構與退化結構。當二元樹的每層節點都被填滿時,達到“完美二元樹”;而當所有節點都偏向一側時,二元樹退化為“鏈結串列”。
|
||
|
||
- 完美二元樹是理想情況,可以充分發揮二元樹“分治”的優勢。
|
||
- 鏈結串列則是另一個極端,各項操作都變為線性操作,時間複雜度退化至 $O(n)$ 。
|
||
|
||
![二元樹的最佳結構與最差結構](binary_tree.assets/binary_tree_best_worst_cases.png)
|
||
|
||
如下表所示,在最佳結構和最差結構下,二元樹的葉節點數量、節點總數、高度等達到極大值或極小值。
|
||
|
||
<p align="center"> 表 <id> 二元樹的最佳結構與最差結構 </p>
|
||
|
||
| | 完美二元樹 | 鏈結串列 |
|
||
| --------------------------- | ------------------ | ------- |
|
||
| 第 $i$ 層的節點數量 | $2^{i-1}$ | $1$ |
|
||
| 高度為 $h$ 的樹的葉節點數量 | $2^h$ | $1$ |
|
||
| 高度為 $h$ 的樹的節點總數 | $2^{h+1} - 1$ | $h + 1$ |
|
||
| 節點總數為 $n$ 的樹的高度 | $\log_2 (n+1) - 1$ | $n - 1$ |
|