/**

  * 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;

     }

 }