leecode/problems/226.invert-binary-tree.md

## 题目地址
https://leetcode.com/problems/invert-binary-tree/description/

## 题目描述

```
Invert a binary tree.

Example:

Input:

     4
   /   \
  2     7
 / \   / \
1   3 6   9
Output:

     4
   /   \
  7     2
 / \   / \
9   6 3   1
Trivia:
This problem was inspired by this original tweet by Max Howell:

Google: 90% of our engineers use the software you wrote (Homebrew), but you can’t invert a binary tree on a whiteboard so f*** off.
```

## 思路
遍历树（随便怎么遍历），然后将左右子树交换位置。
## 关键点解析

- 递归简化操作
- 如果树很高，建议使用栈来代替递归
- 这道题目对顺序没要求的，因此队列数组操作都是一样的，无任何区别

## 代码

* 语言支持：JS，Python，C++

Javascript Code:

```js
/*
 * @lc app=leetcode id=226 lang=javascript
 *
 * [226] Invert Binary Tree
 *
 * https://leetcode.com/problems/invert-binary-tree/description/
 *
 * algorithms
 * Easy (57.14%)
 * Total Accepted:    311K
 * Total Submissions: 540.6K
 * Testcase Example:  '[4,2,7,1,3,6,9]'
 *
 * Invert a binary tree.
 *
 * Example:
 *
 * Input:
 *
 *
 * ⁠    4
 * ⁠  /   \
 * ⁠ 2     7
 * ⁠/ \   / \
 * 1   3 6   9
 *
 * Output:
 *
 *
 * ⁠    4
 * ⁠  /   \
 * ⁠ 7     2
 * ⁠/ \   / \
 * 9   6 3   1
 *
 * Trivia:
 * This problem was inspired by this original tweet by Max Howell:
 *
 * Google: 90% of our engineers use the software you wrote (Homebrew), but you
 * can’t invert a binary tree on a whiteboard so f*** off.
 *
 */
/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {TreeNode}
 */
var invertTree = function(root) {
  if (!root) return root;
  // 递归
  //   const left = root.left;
  //   const right = root.right;
  //   root.right = invertTree(left);
  //   root.left = invertTree(right);
  // 我们用stack来模拟递归
  // 本质上递归是利用了执行栈，执行栈也是一种栈
  // 其实这里使用队列也是一样的，因为这里顺序不重要

  const stack = [root];
  let current = null;
  while ((current = stack.shift())) {
    const left = current.left;
    const right = current.right;
    current.right = left;
    current.left = right;
    if (left) {
      stack.push(left);
    }
    if (right) {
      stack.push(right);
    }
  }
  return root;
};
```

Python Code:

```python
# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def invertTree(self, root: TreeNode) -> TreeNode:
        if not root:
            return None
        stack = [root]
        while stack:
            node = stack.pop(0)
            node.left, node.right = node.right, node.left
            if node.left:
                stack.append(node.left)
            if node.right:
                stack.append(node.right)
        return root
```
C++ Code：
```C++
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* invertTree(TreeNode* root) {
        if (root == NULL) return root;
        auto q = queue<TreeNode*>();
        q.push(root);
        while (!q.empty()) {
            auto n = q.front(); q.pop();
            swap(n->left, n->right);
            if (n->left != nullptr) {
                q.push(n->left);
            }
            if (n->right != nullptr) {
                q.push(n->right);
            }
        }
        return root;
    }
};
```