public void insert (double item) {  
    if (first == null || first.item >= item) {
      first = new Node (item, first);
    } else {
      insertH (first, item);
    }
  }
  private static void insertH (Node x, double item) { 
    if (x.next == null || x.next.item >= item) {
      x.next = new Node (item, x.next);
    }
  }