AdventOfCodeDay10.cs

void Main()
{
	var input = "1113222113";

	Enumerable.Range(1, 50).ToList().ForEach(i =>
	{
		input = Parse(input);
	});
	Console.WriteLine($"{input.Length}");
}

private string Parse(string input)
{
	var counter = 0;
	var ret = new StringBuilder();
	var prevChar = input.First();
	foreach (var c in input)
	{
		if (c == prevChar)
		{
			counter++;
		}
		else
		{
			ret.Append(counter.ToString());
			ret.Append(prevChar);

			counter = 1;
		}
		prevChar = c;
	}
	ret.Append(counter.ToString());
	ret.Append(prevChar);

	return ret.ToString();
}