[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;    }};