leetcode - symmetric binary tree

Title description:
Given a binary tree, check if it is mirror-symmetric.
For example, the binary tree [1,2,2,3,4,4,3] is symmetric.

1

/
2 2
/ \ /
3 4 4 3

class Solution {public boolean isSymmetric(TreeNode root) {if(root == null) return true;return isSymmetricChild(root.left,root.right)span>;}public boolean isSymmetricChild(TreeNode s,TreeNode t) {if(s == null && t == nullspan>) return true;if(s== null && t != null || s != null && t == null) return false;if(s.val != t.val) return false;return isSymmetricChild(s.left,t.right)span>&&isSymmetricChild(s.right,t.left);}}