1
var isSymmetric = function (root) {
2
  return helper(root.left, root.right);
3

4
  function helper(left, right) {
5
    if (!left && !right) return true;
6
    if ((!left && right) || (left && !right) || left.val !== right.val)
7
      return false;
8
    return helper(left.left, right.right) && helper(left.right, right.left);
9
  }
10
};