LeetCode_101Symmetric Tree

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).
For example, this binary tree is symmetric:
1
/ \
2 2
/ \ / \
3 4 4 3
But the following is not:
1
/ \
2 2
\ \
3 3

public boolean isSymmetric(TreeNode root) {            if (root ==null) return true;            return symmetric(root.left, root.right);     }     private boolean symmetric(TreeNode left, TreeNode right){         if (left == null && right ==null) return true;         if (left !=null && right !=null &&right.val == left.val) {            return symmetric(left.right, right.left)&&symmetric(left.left, right.right);        }else {            return false;        }     }