using System;

using System.Collections.Generic;

using System.Linq;

public static class Program

{

    public static void Main(string[] args)

    {

        var iter = 10;

        IEnumerable<int> s = new[] { 1 };

        for (int i = 0; i < iter; i++)

        {

            Console.WriteLine(string.Join(string.Empty, s));

            s = NextSeries(s).ToList();

        }

    }

​

    private static IEnumerable<int> NextSeries(IEnumerable<int> source)

    {

        int? current = null;

        int count = 0;

        foreach (var e in source)

        {

            if (current == null)

            {

                current = e;

                count = 1;

                continue;

            }

            if (current.Value == e)

            {

                count++;

            }

            else

            {

                yield return current.Value;

                yield return count;

                current = e;

                count = 1;

            }

        }

        if (current != null)

        {

            yield return current.Value;

            yield return count;

        }

    }

}

​