using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var n5 = new Node("node5");

        var n4 = new Node("node4")

        {Next = n5, };

        var n3 = new Node("node3")

        {Next = n4, };

        var n2 = new Node("node2")

        {Next = n3, };

        var n1 = new Node("node1")

        {Next = n2, };

        Console.WriteLine( string.Join(" => ", EnumerateNodes(n1).Select(e => e.Name)) );

        var inputnode = n3;

        var outputnode = mystery( inputnode );

        Console.WriteLine( string.Join(" => ", EnumerateNodes( outputnode ).Select(e => e.Name)) );

    }

​

    static IEnumerable<Node> EnumerateNodes(Node node)

    {

        var current = node;

        while (current != null)

        {

            yield return current;

            current = current.Next;

        }

    }

​

    static Node mystery(Node ptr)

    {

        Node temp;

        Node previous = null;

        while (ptr != null)

        {

            temp = ptr.next;

            ptr.next = previous;

            previous = ptr;

            ptr = temp;

        }

​

        return previous;

    }

}

​

class Node

{

    public Node(string name)

    {

        Name = name;

    }

​

    public string Name

    {

        get;

    }

​

    public Node Next

    {

        get;

        set;

    }

    public Node next { get => Next; set => Next = value; }

}