using System;

​

public class ListNode

{

    public int Value;

    public ListNode Next;

    public ListNode(int value) { Value = value; }

}

​

public class Program

{

    static bool HasCycle(ListNode head)

    {

        if (head == null)

            return false;

​

        ListNode walker = head;

        ListNode runner = head;

​

        while (runner.Next != null && runner.Next.Next != null)

        {

            walker = walker.Next;

            runner = runner.Next.Next;

            if (walker == runner) return true;

        }

        return false;

    }

    static ListNode DetectCycle(ListNode head)

    {

        if (head == null)

            return null;

​

        ListNode walker = head;

        ListNode runner = head;

​

        while (runner.Next != null && runner.Next.Next != null)

        {

            walker = walker.Next;

            runner = runner.Next.Next;

​

            if (walker == runner)

            {

                ListNode tempWalker = head;

                while (tempWalker != walker)

                {

                    walker = walker.Next;

                    tempWalker = tempWalker.Next;

                }

                return walker;

            }

        }

        return null;    

​

    }

    public static void Main()

    {

        ListNode node1 = new ListNode(1);

        ListNode node2 = new ListNode(2);

        ListNode node3 = new ListNode(3);

        ListNode node4 = new ListNode(4);

        node1.Next = node2;

        node2.Next = node3;

        node3.Next = node4;

        node4.Next = node2;

        bool hasCycle = HasCycle(node1);

        if(hasCycle)

        {

            Console.WriteLine(DetectCycle(node1).Value);

        }

    }

}