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;
}
};