using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        List<int> myList1 = null;

        List<int> myList2 = new() {  };

        List<int> myList3 = new() { 1 };

        List<int> myList4 = new() { 1, 1 };

        List<int> myList5 = new() { 1, 2 };

        List<int> myList6 = new() { 1, 2, 1 };

        List<int> myList7 = new() { 1, 1, 2, 1 };

        List<int> myList8 = new() { 1, 2, 1, 1 };

        List<int> myList9 = new() { 1, 1, 9, 2, 2, 3, 9, 4, 5, 5, 1, 9, 9 };

        List<List<int>> lists = new() { myList1, myList2, myList3, myList4, myList5, myList6, myList7, myList8, myList9 };

        foreach (var list in lists)

        {

            var filteredList = GetFilteredList(list);

            Console.WriteLine("Original list: " + (list == null ? "null" : "{ " + string.Join(", ", list) + " }"));

            Console.WriteLine("Filtered list: " + (filteredList == null ? "null" : "{ " + string.Join(", ", filteredList) + " }"));

            Console.WriteLine();

        }

    }

    public static List<int> GetFilteredList(List<int> list)

    {

        // if list is null; return null

        if (list == null)

        {

            return null;

        }

        // If list is empty or contains only one element; return list as new list

        if (!list.Skip(1).Any())

        {

            return list.ToList();

        }

        var filtered = list.AsEnumerable();

        // Remove duplicates at beginning of list (if any)

        if (list.First() == list.Skip(1).First())

        {

            filtered = filtered.SkipWhile(l => l == list.First());

        }

        // Remove duplicates at end of list (if any)

        if (list.Last() == list.SkipLast(1).Last())

        {

            filtered = filtered.SkipLastWhile(l => l == list.Last());

        }

        return filtered.ToList();

    }

}

​

public static class Helper

{

    public static IEnumerable<TSource> SkipLastWhile<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)

    {

        var buffer = new List<TSource>();

​

        foreach (var item in source)

        {

            if (predicate(item))

            {

                buffer.Add(item);

            }

            else

            {

                if (buffer.Count > 0)

                {

                    foreach (var bufferedItem in buffer)

                    {

                        yield return bufferedItem;

                    }

​

                    buffer.Clear();

                }

​

                yield return item;

            }

        }

    }

}