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.

/**
* 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 tmp = root.left;
　　　　root.left = root.right;
　　　　root.right = tmp;
　　　　invertTree(root.left);
　　　　invertTree(root.right);
　　　　return root;
　　}
}