LeetCode:Invert Binary Tree

Invert a binary tree.

     4
   /   
  2     7
 /    / 
1   3 6   9

     4
   /   
  7     2
 /    / 
9   6 3   1

Trivia:
This problem was inspired by this original tweet by Max Howell:

Google: 90% of our engineers use the software you wrote (Homebrew), but you can’t invert a binary tree on a whiteboard so fuck off.

 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         TreeNode *result=root;
14         
15         if(root==NULL) return NULL;
16         invertTreeNode(root);
17         invertTree(root->left);
18         invertTree(root->right);
19        
20         return result;
21         
22     }
23     
24     void invertTreeNode(TreeNode *node)
25     {
26         if(node==NULL) 
27             return;
28         TreeNode *temp=node->left;
29         node->left=node->right;
30         node->right=temp;
31     }
32 };