Symmetric Tree

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public boolean isSymmetric(TreeNode root) {        if (root == null) {            return true;        }        return isSymmetricHelper(root.left, root.right);    }        public boolean isSymmetricHelper(TreeNode left, TreeNode right) {        if (left == null && right == null) {            return true;        } else if (left != null && right != null) {            if (left.val != right.val) {                return false;            }            return isSymmetricHelper(left.left, right.right) && isSymmetricHelper(left.right, right.left);        } else {            return false;        }    }}