RunLength. O(n)

RunLength.cs

static void Main(string[] args)
{
    var input = "wwwwaaadexxxxxxywww";

    Console.WriteLine(input);

    for (var i = 0; i < input.Length - 1; i++)
    {
        var count = 1;
        while(i < input.Length - 1
              && input[i] == input[i + 1])
        {
            count++;
            i++;
        }

        Console.Write($"{count}{input[i]} "); //4w 3a 1d 1e 6x 1y 3w
    }
}

           var input = "bbooppqqtghnvvvoopgggggggggggl";
            var result = new StringBuilder();
            var count = 1;

            for (var i = 0; i < input.Length; i++)
            {
                if (i == input.Length - 1)
                {
                    result.Append(input[i]);
                    result.Append(count);
                    break;
                }

                if (input[i + 1] == input[i])
                    count++;
                else
                {
                    result.Append(input[i]);
                    result.Append(count);
                    count = 1;
                }

            }

            Console.WriteLine(result.ToString().Length > input.Length ? input : result.ToString());