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# 面试题27

请完成一个函数，输入一个二叉树，该函数输出它的镜像。

```
例如输入：

     4
   /   \
  2     7
 / \   / \
1   3 6   9
镜像输出：

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



示例 1：

输入：root = [4,2,7,1,3,6,9]
输出：[4,7,2,9,6,3,1]
```

## 限制：

* 0 <= 节点个数 <= 1000

## 注意：本题与主站 226 题相同：<https://leetcode-cn.com/problems/invert-binary-tree/>

## Solutions

* See problem 226 for more solutions.
* **recursion**

```cpp
/**
 * 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* mirrorTree(TreeNode* root) {
        if (!root) return nullptr;
        TreeNode * right = root->right;
        root->right = mirrorTree(root->left);
        root->left = mirrorTree(right);
        return root;
    }
};
```

1. **iteration**
2. preorder/inorder/postorder/levelorder traversal