/**

* 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;

// this line can be removed

if(root.left == null && root.right == null) return root;

TreeNode left = invertTree(root.left);

TreeNode right = invertTree(root.right);

root.left = right;

root.right = left;

return root;

}

}