二叉树的镜像

输入一个二叉树，将它变换为它的镜像。

 

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/**  * 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:     void mirror(TreeNode* root) {         if (!root) return;         mirror (root->left);         mirror (root->right);         swap (root->left, root->right);     } };