def reverse(llist: DoublyLinkedListNode): DoublyLinkedListNode = {
      def reverseNode(node: DoublyLinkedListNode, next: DoublyLinkedListNode): DoublyLinkedListNode =
        if (node == null) next
        else {
          val prev = node.next
          node.prev = prev
          node.next = next
          reverseNode(prev, node)
        }
      
      reverseNode(llist, null)
    }