Invert Binary Tree

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     TreeNode* invertTree(TreeNode* root) {
13         if (root == NULL) {
14             return root;
15         }
16         auto tmp = invertTree(root->right);
17         root->right = invertTree(root->left);
18         root->left = tmp;
19     }
20 };