List<string> str = new List<string> {"a","b","a","a","c","b","c","c","d","c"};

                var tt = from ps in str 

                    group ps by ps 

                    into g

                    select new {g.Key,num=g.Count()};

                var newstr = tt.OrderByDescending(p => p.num);

                foreach (var item in newstr)

                {

                    Console.WriteLine(item.Key);

                }