using System;

​

public class Program

{

    public static void Main(string[] args)

    {

        var first = new Node(1);

        var second = new Node(2);

        var third = new Node(3);

        var fourth = new Node(4);

        var fifth = new Node(5);

        var sixth = new Node(6);

        first.Next = second;

        second.Next = third;

        third.Next = fourth;

        fourth.Next = fifth;

        fifth.Next = sixth;

        first.Print();

        PairWiseSwap(ref first);

        Console.WriteLine("=============== After swapping ===============");

        first.Print();

    }

​

    static void PairWiseSwap(ref Node head)

    {

        var current = head;

        Node prev = null;

        if (current != null && current.Next != null)

        {

            head = current.Next;

        }

​

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

        {

            var next = current.Next;

            current.Next = next.Next;

            next.Next = current;

            if (prev != null)

            {

                prev.Next = next;

            }

​

            prev = current;

            current = current.Next;

        }

    }

}

​

public class Node

{

    public int Data

    {

        get;

        set;

    }

​

    public Node Next

    {

        get;

        set;

    }

​

    public Node(int data)

    {

        this.Data = data;

        this.Next = null;

    }

​

    public void Print()

    {

        var node = this;

        do

        {

            Console.WriteLine(node.Data);

            node = node.Next;

        }

        while (node != null);

    }

​

    public int GetLengthIterative()

    {

        var node = this;

        var count = 0;

        while (node != null)

        {

            count++;

            node = node.Next;

        }

​

        return count;

    }

​

    public int GetLengthRecursive()

    {

        if (this.Next == null)

        {

            return 1;

        }

​

        return 1 + this.Next.GetLengthRecursive();

    }

​

    public override string ToString()

    {

        return Data.ToString();

    }

}