using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("AAAABBUUYYYYTTTTTTTT".CountMap());

    }

}

​

public static class Extensions

{

    public static string CountMap(this string value)

    {

        return  

            string.Join(

                string.Empty,         

                Enumerable.Range(0, value.Length) 

                .Where(i => (i == 0 || value[i] != value[i - 1])) 

                .Select(i => value.ToCharArray().Skip(i).TakeWhile(c => c == value[i])) 

                .Select(a => string.Concat(a.First(), a.Count())) 

            ); 

    }

}

​