Invert a binary tree.

Example:

Input:

     4
   /   \   2     7
 / \   / \ 1   3 6   9
Output:

     4
   /   \   7     2
 / \   / \ 9   6 3   1

先序遍历并交换左右的元素。

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