/**

* Definition for a binary tree node.

* public class TreeNode {

* int val;

* TreeNode left;

* TreeNode right;

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

* }

*/

class Solution {

public boolean isSymmetric(TreeNode root) {

if (root == null) return true;

return isSymmetric(root.left, root.right);

}

public boolean isSymmetric(TreeNode node1, TreeNode node2) {

if (node1 == null && node2 == null) return true;

if ((node1 == null) ^ (node2 == null)) return false;

if (node1.val != node2.val) return false;

return isSymmetric(node1.left, node2.right) && isSymmetric(node1.right, node2.left);

}

}