public boolean isSymmetric_help(TreeNode left, TreeNode right) {
		if (left == null)
			return right == null;
		if (right == null)
			return left == null;
		if (right.val != left.val)
			return false;
		if (!isSymmetric_help(left.left, right.right))
			return false;
		if (!isSymmetric_help(left.right, right.left))
			return false;

		return true;
	}