AdventOfCodeDay10.cs

using System; 
using System.Linq;
using System.Text; 

namespace AdventOfCodeDay10
{
	class Program
	{
		static void Main(string[] args)
		{
			var input = "1321131112";
			for (var i = 0; i < 50; i++)
			{
				input = LookAndSay(input);
				if (i + 1 == 40 || i + 1 == 50)
				{
					Console.WriteLine($"{i + 1} : {input.Length}");
				}
			}
			Console.ReadLine();
		}

		private static string LookAndSay(string input)
		{
			var sb = new StringBuilder();
			var current = input.First();
			var count = 0;
			foreach (var c in input)
			{
				if (c == current)
				{
					count++;
				}
				else
				{
					sb.Append($"{count}{current}");
					current = c;
					count = 1;
				}
			}
			sb.Append($"{count}{current}");

			return sb.ToString();
		}
	}
}