using System;

using System.Text;

public class Program

{

    public static void Main()

    {

        var testCases = new []

        {

            "aaabccccdd", // expected: "a3bc4d2"

            "yyyyyzzzzzzzzzz", // expected: "y5z10"

            "abcd", // expected: "abcd"

            "hhhccchhhccc", // expected: "h3c3h3c3"

            "a", // expected: "a"

            "aa", // expected "a2"

            "", // expected: ""

            "a   ", // expected: "a 3"

            null // expected: ""

        };

        for(var i = 0; i < testCases.Length; i++)

        {

            var testCase = testCases[i];

            Console.WriteLine($"--------- {i+1} ---------");

            Console.WriteLine($"Input: {testCase}");

            Console.WriteLine($"Output: {Compress(testCase)}");

            Console.WriteLine();

        }       

    }

    private static string Compress(string inputString)

    {

        var result = new StringBuilder();

​

        char? lastChar = null;

        var lastCharCount = 0;

        void compressLastChar()

        {

            if(lastChar != null)

            { 

                result.Append(lastChar);

                if(lastCharCount > 1)

                {

                    result.Append(lastCharCount);

                } 

            }

        }

​

        foreach(char currentChar in inputString ?? "")

        {

            if(currentChar != lastChar)

            { 

                compressLastChar();

                lastCharCount = 0;

            }

            lastChar = currentChar;

            lastCharCount++;

        }

​

        compressLastChar();

        return result.ToString();

    }

}