/**

  * Definition for a binary tree node.

  * public class TreeNode {

  *     int val;

  *     TreeNode left;

  *     TreeNode right;

  *     TreeNode(int x) { val = x; }

  * }

  */

 class Solution {

     public TreeNode invertTree(TreeNode root) {

         if(root==null){

             return null;

         }

         TreeNode temp= root.left;

         root.left=root.right;

         root.right=temp;

         invertTree(root.left);

         invertTree(root.right);

         return root;

     }

 }