[leetcode] 226.Invert Binary Tree

题目：

Invert a binary tree.

4

/ \

2 7

/ \ / \

1 3 6 9

to

4
/ \

7 2

/ \ / \

9 6 3 1

题意：

翻转二叉树。

思路：

这道题采用递归的思想，将左右子树分别反转，然后将左右子树对调。

代码如下：

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