示例：
​
输入：
​
     4
   /   \
  2     7
 / \   / \
1   3 6   9
输出：
​
     4
   /   \
  7     2
 / \   / \
9   6 3   1

/**
 * 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) return NULL;
        root->right=invertTree(root->right);
        root->left=invertTree(root->left);
        TreeNode* tmp=root->right;
        root->right=root->left;
        root->left=NULL;
        root->left=tmp;
        return root;
​
    }
};