using System;

public class Program

{

    public static void Main()

    {

        Node root2 = new Node(1);

        root2.next = new Node(2);

        root2.next.next = new Node(3);

        root2.next.next.next = new Node(4);

        root2.next.next.next.next = new Node(5);

        Node r = InvertPairs(root2);

        Print(r);

    }

    public static void Print(Node n) {

        while(n != null) {

            Console.Write(n.data + " -> ");

            n = n.next;

        }

        Console.WriteLine("\n");

    }

    public static Node InvertPairs(Node head) {

        Node newList = null;

        while(head != null && head.next != null) {

            if (newList == null) {

                newList = head.next;

            }

            Node n = head.next;

            Node newHead = n.next;

            n.next = head;

            head.next = newHead;

            if (newHead != null && newHead.next != null) {

                head.next = newHead.next;

            }

            head = newHead;

        }

        return newList;

    }

}

​

​

public class Node {

    public int data;

    public Node next;

    public Node (int i) {

        data = i;

    }

}