public int product (int m, int n)
  {
    if (m == n) return m;
    int middle = (m+n) / 2;
    return product (m, middle) * product (middle+1, n);
  }

  // reversing Lists recursively

  public ListNode concat2 (ListNode node1, ListNode node2)
  {
    if (node1 == null) return node2;
    node1.next = concat2 (node1.next, node2);
    return node1;
  }

  public ListNode reverse_helper (ListNode node)
  {
    if (node == null) return null;

    ListNode tail = node.next;

    node.next = null;
    return concat2 (reverse_helper(tail), node);
  }

  public void reverse2 ()
  {
    head = reverse_helper (head);
  }