[leetcode 226]Invert Binary Tree

Invert a binary tree.

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

to

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

Trivia:

This problem was inspired by this original tweet by Max Howell:

Google: 90% of our engineers use the software you wrote (Homebrew), but you can’t invert a binary tree on a whiteboard so fuck off.

[Solution]

交换左右子树，然后递归交换即可。

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     TreeNode* invertTree(TreeNode* root) 
13     {
14         if (root == NULL)
15             return root;
16         TreeNode *swap_node = root->left;
17         root->left = root->right;
18         root->right = swap_node;
19         root->left = invertTree(root->left);
20         root->right = invertTree(root->right);
21         
22         return root;
23     }
24 };