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