LeetCode OJ 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 sofuck off.

用递归解决还是很简单的，上代码：

 /**
  * Definition for a binary tree node.
  * public class TreeNode {
  *     int val;
  *     TreeNode left;
  *     TreeNode right;
  *     TreeNode(int x) { val = x; }
  * }
  */
 public class Solution {
     public TreeNode invertTree(TreeNode root) {
         if(root == null) return null;

         TreeNode left = invertTree(root.left);
         TreeNode right = invertTree(root.right);

         root.left = right;
         root.right = left;
         return root;
     }
 }