RLE.cs

using System;
using System.Globalization;
using System.Linq;
using System.Text;
using Microsoft.VisualStudio.TestTools.UnitTesting;

namespace MerryChristmas
{
    public static class RLE
    {
        public static string Encode(string input)
        {
            if (input == null)
                throw new ArgumentNullException(nameof(input));
            if (input == "")
                return "";
            var output = new StringBuilder();
            int count = 1;
            for (int i = 0; i < input.Length; i++)
            {
                var ch = input[i];
                if (i < input.Length - 1 && ch == input[i + 1])
                {
                    count++;
                }
                else
                {
                    output.Append(count.ToString(CultureInfo.InvariantCulture));
                    output.Append(ch);
                    if (count != 1)
                        count = 1;
                }
            }
            return output.ToString();
        }

        public static string Decode(string input)
        {
            if (input == null)
                throw new ArgumentNullException(nameof(input));
            if (input == "")
                return "";
            var output = new StringBuilder();
            for (int i = 0; i < input.Length; i++)
            {
                int len = 0;
                while (char.IsDigit(input[i + len]))
                    len++;
                
                int count = Convert.ToInt32(input.Substring(i, len), CultureInfo.InvariantCulture);
                i += len;
                for (int x = 0; x < count; x++)
                    output.Append(input[i]);
            }
            return output.ToString();

        }
    }

    [TestClass]
    public class UnitTests
    {
        [TestMethod]
        public void TestEncodeDecode()
        {
            var testCases = new[]
            {
                ("", ""),
                ("AAAABBBCCDAA", "4A3B2C1D2A"),
                ("A", "1A"),
                ("AAAAAAAAAA", "10A")
            };

            foreach (var testCase in testCases)
            {
                Assert.AreEqual(testCase.Item2, RLE.Encode(testCase.Item1));
                Assert.AreEqual(testCase.Item1, RLE.Decode(testCase.Item2));
            }
        }
    }
}