public static void main(String args[]) {
   double_ll<Integer> dll = new double_ll<Integer>(10);
   dll.insert(20);
   dll.insert(30);
   dll.insert(40);
   dll.print();
   dll = reverse_dll(dll);
   dll.print();
 }
  public static double_ll reverse_dll(double_ll dll) {
    if (dll.isEmpty()) {
      System.out.println("empty lst");
      return null;
    }

    dll_node prev = null;
    dll_node curr = dll.head;
    while (curr != null) {

      curr.prev = curr.next;
      curr.next = prev;
      prev = curr;
      curr = curr.prev;
    }

    if (prev != null) {
      dll.head = prev;
    }

    return dll;
  }