Parsing string (count char) (codingame problem

            string s = "aaaabbbddddaaa";

            //s = "abc";

            string output = "";
            char c = s[0];
            int count = 0;
            for (int i = 0; i < s.Length; i++)
            {
                if (c == s[i])
                {
                    count++;
                }
                else
                {
                    output += count.ToString() + c;
                    c = s[i];
                    count = 1;
                }
            }

            // include last char
            if (output.Length > 0 && output[output.Length - 1] != c)
            {
                output += count.ToString() + c;
            }

            Console.WriteLine(output); // output: 4a3b4d3a