Invert Binary Tree

题目：

Invert a binary tree.

     4
   /   
  2     7
 /    / 
1   3 6   9

     4
   /   
  7     2
 /    / 
9   6 3   1

解析：

 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         {
15             TreeNode* tmp=root->left;
16             root->left=root->right;
17             root->right=tmp;
18             invertTree(root->left);
19             invertTree(root->right);
20         }
21         return root;
22     }
23 };