[LeetCode226]Invert Binary Tree

题目：

Invert a binary tree.

     4
   /     2     7
 / \   / 1   3 6   9

to

     4
   /     7     2
 / \   / 9   6 3   1

反转二叉树，左右儿子值交换

代码：

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