using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var hashSet = new HashSet<int>(2);

        Console.WriteLine(hashSet.Contains(2));

    }

    public ListNode ReverseList(ListNode head) {

        helper(head, out var newHead);

        return newHead;

    }

    private ListNode helper(ListNode current, out ListNode newHead) {

        newHead = null;

        if(current == null) {

            return null;

        }

        var next = helper(current.next, out newHead);

        if(next == null) {

            newHead = current;

        } else {

            next.next = current;

        }

        return current;

    }

}

​

public class ListNode {

    public int val;

    public ListNode next;

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

        this.val = val;

        this.next = next;

    }

}