思路：递归

/*** 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==NULL) return NULL;TreeNode *right=mirrorTree(root->left);TreeNode *left=mirrorTree(root->right);root->right=right;root->left=left;return root;}
};