public Node<Integer> addHelper(Node<Integer> head1, Node<Integer> head2) {
		if (head1 == null && head2 == null)
			return null;
		int sum = 0;
		if (head1 != null)
			sum += head1.data;
		if (head2 != null)
			sum += head2.data;
		Node<Integer> next1 = (head1 == null) ? null : head1.next;
		Node<Integer> next2 = (head2 == null) ? null : head2.next;

		Node<Integer> rest = addHelper(next1, next2);

		if (rest != null && rest.data >= 10) {
			rest.data -= 10;
			sum += 1;
		}
		Node<Integer> head = new Node<Integer>(sum, rest);
		return head;
	}