using System;

using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

        var test1 = "aaabccccdd";

        var test2 = "yyyyyzzzzzzzzzz";

        var test3 = "abcd";

        var test4 = "hhhccchhhccc";

        Console.WriteLine(ProcessStringCompression(test1));

        Console.WriteLine(ProcessStringCompression(test2));

        Console.WriteLine(ProcessStringCompression(test3));

        Console.WriteLine(ProcessStringCompression(test4));

    }

​

    public static string ProcessStringCompression(string stringToCompress)

    {

        if (stringToCompress == null)

        {

            throw new ArgumentNullException("stringToCompress");

        }

​

        if (!Regex.IsMatch(stringToCompress, @"^[a-zA-Z]+$"))

        {

            throw new ArgumentException("Only non numeric alpha characters supported.");

        }

​

        if (stringToCompress.Length < 2)

        {

            return stringToCompress;

        }

​

        var charArray = stringToCompress.ToCharArray();

        var indexAt = 0;

        var i = 0;

        while (i < charArray.Length)

        {

            var j = i + 1;

            var counter = 1;

            while (j < charArray.Length && charArray[j] == charArray[i])

            {

                i++;

                j++;

                counter++;

            }

​

            charArray[indexAt] = charArray[i];

            if (counter > 1)

            {

                var countAsString = counter.ToString();

                foreach (var theChar in countAsString)

                {

                    charArray[++indexAt] = theChar;

                }

            }

​

            indexAt++;

            i = j;

        }

​

        return new string (charArray, 0, indexAt);

    }

}