using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

        public List<Card> FullHouse(List<Card> cards)

        {

            var _3pair = GetHighestPair(cards, 3); //triple returned

            if (_3pair != null)

            {

                var _2pair = GetHighestPair(Remaining(cards, _3pair), 2);

                if (_2pair != null)

                    return new List<Card>(_3pair.Concat(_2pair));

            }

            return null;

        }

​

        private List<Card> GetHighestPair(List<Card> cards, int size) => cards.GroupBy(c => c.Value)

                                                                               .OrderByDescending(grp => grp.Key)

                                                                               .FirstOrDefault(grp => grp.Count() >= size)?

                                                                               .Take(size).ToList();

​

        private List<Card> Remaining(List<Card> toConsider, List<Card> toIgnore) => toConsider.Where(c => toIgnore.Contains(c) == false).ToList();

}

​