using System;

using System.Collections.Generic;

using System.Collections;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

         ListNode head= new ListNode(1, new ListNode(2));

        head.next.next=new ListNode(2, new ListNode(1));

​

        List<ListNode> list=new List<ListNode>();

        while(head!=null){

            list.Add(head);

            head=head.next;

        }

        List<ListNode> list2=new List<ListNode>(list);

        list2.Reverse();

        foreach(var item in list){

        Console.Write(item.val);    

        }

        Console.WriteLine();

        foreach(var item in list2){

        Console.Write(item.val);    

        }

    }

    public class ListNode {

      public int val;

      public ListNode next;

      public ListNode(int val=0, ListNode next=null) {

          this.val = val;

          this.next = next;

      }

  }

​

}

​

​