using System;

​

public class Program

{

    public static void Main()

    {    

        List<string> newList = new List<string>();

        newList.addNode("A");

        newList.addNode("B");

        newList.addNode("D");

        newList.addNode("D");

​

        Console.WriteLine("This list has: " + newList.getListCount().ToString()+" elements:");

        newList.showList();

​

        Console.WriteLine("Replacing element 3: D on C");

        newList.replaceNode(3, "C");

        Console.WriteLine("This list has: " + newList.getListCount().ToString() + " elements:");

        newList.showList();

​

        Console.WriteLine("Creating new list");

        List<string> newList2 = new List<string>();

        newList2.addNode("E");

        newList2.addNode("F");

        newList2.addNode("H");

​

        Console.WriteLine("This list has: " + newList2.getListCount().ToString() + " elements:");

        newList2.showList();

​

        Console.WriteLine("Merging two lists");

        newList.mergeTwoLists(newList2);

​

        Console.WriteLine("This list has: " + newList.getListCount().ToString() + " elements:");

        newList.showList();

​

        Console.WriteLine("Replacing last element: H on G");

        newList.replaceNode(7, "G");

​

        Console.WriteLine("This list has: " + newList.getListCount().ToString() + " elements:");

        newList.showList();

    }

}

​

class ListNode<T>

{

    public readonly T Value;

    public readonly ListNode<T> Next;

​

    public ListNode(T value, ListNode<T> next)

    {

        this.Value = value;

        this.Next = next;

    }

}

​

class List<T>

{

    private ListNode<T> head;

​

    public void addNode(T value)

    {

        if (head == null)

        {

            head = new ListNode<T>(value, null);

        }

        else

        {

            if (head.Next == null)

            {

                ListNode<T> nextNode = new ListNode<T>(value, null);

                head = new ListNode<T>(head.Value, nextNode);

            }

            else

            {

                ListNode<T> nextNodeHead = head;

                bool final = false, firstHead = false;

                while (true)

                {

                    if (final && firstHead)

                    {

                        head = new ListNode<T>(value, head);

                        break;

                    }

                    if (!firstHead)

                    {

                        nextNodeHead = head.Next.Next;

                        if (nextNodeHead == null)

                        {

                            final = true;

                        }

                        ListNode<T> buf = new ListNode<T>(head.Value, null);

                        head = new ListNode<T>(head.Next.Value, buf);

                        firstHead=true;

                    }

                    else

                    {

                        head = new ListNode<T>(nextNodeHead.Value, head);

                        nextNodeHead = nextNodeHead.Next;

                        if (nextNodeHead == null)

                        {

                            final = true;

                        }

                    }

                }

                firstHead = false;

                while (true)

                {

                    if (!firstHead)

                    {

                        ListNode<T> buf = new ListNode<T>(head.Value, null);

                        nextNodeHead = head.Next.Next;

                        head = new ListNode<T>(head.Next.Value, buf);

                        firstHead = true;

                    }

                    else

                    {

                        ListNode<T> buf = nextNodeHead;

                        nextNodeHead = nextNodeHead.Next;

                        head = new ListNode<T>(buf.Value, head);

                        if (nextNodeHead == null) break;

​

                    }

                }

            }

        }

    }

​

    public void replaceNode(int index, T value)

    {

        if (index <= this.getListCount())

        {

            bool firstHead = false;

            ListNode<T> nextNodeHead = head;

            for (int i = 1; i < index; i++)

            {

                if (!firstHead)

                {

                    nextNodeHead = head.Next.Next;

                    ListNode<T> buf = new ListNode<T>(head.Value, null);

                    head = new ListNode<T>(head.Next.Value, buf);

                    firstHead = true;

                }

                else 

                {

                    head = new ListNode<T>(nextNodeHead.Value, head);

                    nextNodeHead = nextNodeHead.Next;

                }    

            }

            if (!firstHead) 

            { 

                head = new ListNode<T>(value, head.Next);

                return;

            }

            ListNode<T> prevNode = nextNodeHead;

            nextNodeHead = head.Next;

            head = new ListNode<T>(value, prevNode);

            firstHead = false;

            while (true)

            {

                head = new ListNode<T>(nextNodeHead.Value, head);

                if (nextNodeHead.Next == null)

                {

                    break;

                }

                else

                {

                    nextNodeHead = nextNodeHead.Next;

                }                    

            }

        }

    }

​

    public int getListCount ()

    {

        ListNode<T> step = head;

        int count = 0;

        while (step != null)

        {

            count++;

            step = step.Next;

        }

        return count;

    }

​

    public void mergeTwoLists (List<T> secondList)

    {

        ListNode<T> step = secondList.head;

        while (step != null)

        {

            this.addNode(step.Value);

            step = step.Next;

        }

    }

    public void showList()

    {

        ListNode<T> step = head;

        while (step != null)

        {

            T nodeValue = step.Value;

            Console.WriteLine(nodeValue);

            step = step.Next;

        }

    }

​

​

}