public static void Main()
string input = "qaaaabbqqqxwwwzztta";
string output = DoWork(input);
Console.WriteLine("Все просто ! - {0}",output);
public static String DoWork(string input)
for(int i=0;i<input.Length;i++)
string element = input[i].ToString();
if (i+1 == input.Length ||(i+1 < input.Length && input[i]!=input[i+1]))
while ( i+counter < input.Length && input[i]==input[i+counter])
output += counter.ToString();