using System;

using System.Collections.Generic;

using System.Linq;

​

public class Solution

{

    /// <summary>

    /// Given the head of a singly linked list swap the order of each pair of elements.

    /// </summary>

    /// <param name="head">The head of the list.</param>

    /// <remarks>

    ///     Example:

    ///         Input: (1) -> (2) -> (3) -> (4) -> (5)

    ///         Output: (2) -> (1) -> (4) -> (3) -> (5)

    /// </remarks>

    public static Node SwapPairs(Node head)

    {      

       return head;

    }

​

    public static void Main(string[] args)

    {

        Console.WriteLine("\nTesting odd input...");

        TestOddInput();

​

        Console.WriteLine("\nTesting even input...");

        TestEvenInput();

​

        Console.WriteLine("\nTesting small input...");

        TestSmallInput();

​

        Console.WriteLine("\nTesting null input...");

        TestNullInput();

    }

​

    public static void TestOddInput()

    {

        Node head = Node.CreateList(5);

        Console.WriteLine("Input: " + head);

​

        head = SwapPairs(head);

        Console.WriteLine("Output: " + head);

    }

​

    public static void TestEvenInput()

    {

        Node head = Node.CreateList(6);

        Console.WriteLine("Input: " + head);

​

        head = SwapPairs(head);

        Console.WriteLine("Output: " + head);

    }

​

    public static void TestSmallInput()

    {

        Node head = new Node(1);

        Console.WriteLine("Input: " + head);

​

        head = SwapPairs(head);

        Console.WriteLine("Output: " + head);

    }

​

    public static void TestNullInput()

    {

        Console.WriteLine("Input: null");

​

        Node head = SwapPairs(null);

        Console.WriteLine("Output: " + (head != null? head.ToString(): "null"));

    }

​

    public class Node

    {

        public Node(int data)

        {

            this.Data = data;

        }

​

        public Node Next { get; set; }

​

        public int Data { get; private set; }

​

        public static Node CreateList(int count)

        {

            List<Node> nodes = Enumerable.Range(1, count).Select(i => new Node(i)).ToList();

            for (int i = 0; i < nodes.Count - 1; i++)

            {

                nodes[i].Next = nodes[i + 1];

            }

​

            return nodes.First();

        }

​

        public override string ToString()

        {

            if (Next != null)

            {

                return string.Format("({0}) -> {1}", Data, Next);

            }

            else

            {

                return string.Format("({0}) -> null", Data);

            }

        }

    }

}

​