using System;

using System.Linq;

using System.Text;

public class Program

{

    public static void Main()

    {

            var theString = "abacadttttssss";

            var qry2 = theString.ToCharArray().GroupBy(c => c).OrderByDescending(a=>a.Key);

            System.Text.StringBuilder stringBuilder = new StringBuilder();

            var newindex = 0;

            foreach (var item in qry2)

            {

                var charCount = item.Count();

                if (charCount < 2) // For non-repeater charactors

                {

                    stringBuilder.Append(item.Key);

                }

                else // insert repeated char at first  if multiple repeatead char exists it will be added in desc. order 

                {

                    stringBuilder.Insert(newindex, string.Join("", item.ToArray()));

                    newindex += charCount;

                }

            }

​

            var finalstring = stringBuilder.ToString();

           Console.WriteLine(finalstring);          

    }

}