using System;

​

public class Program {

    public static void Main()

    {

        Console.WriteLine("aaaa") ;

    }

}

​

​

/**

 * Definition for singly-linked list. */

 public class ListNode {

      public int val;

      public ListNode next;

      public ListNode(int x) { val = x; }

 }

public class Solution {

    public ListNode ReverseList(ListNode head) {

        ListNode prev = null;

        ListNode curr = head;

        while (curr != null) {

            ListNode nextTemp = curr.next;

            curr.next = prev;

            prev = curr;

            curr = nextTemp;

        }

        return prev;

    }

}