LeetCode: Invert Binary Tree

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