LeetCode101

 1 class Solution {
 2     public boolean isSymmetric(TreeNode root) {
 3         if (root == null) {
 4             return true;
 5         }
 6         return symmetric(root.left, root.right);
 7     }
 8     
 9     private boolean symmetric(TreeNode left, TreeNode right) {
10         if (left == null && right == null) {
11             return true;
12         }
13         if (left == null || right == null) {
14             return false;
15         }
16         return left.val == right.val && symmetric(left.left, right.right) && symmetric(left.right, right.left);
17     }
18 }