using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

public class Program

{

    public static void Main()

    {

        var (n, c, sequence) = ABACABA.Calculate(Enumerable.Range('a', 23).Select(c => (char)c).ToArray()).Last(); // 23 letter is W, Z is too long for .NET Fiddle

        Console.WriteLine($"{n:00}({c}): {sequence}");

        Print('0', '9');

        Print('a', 'z'); // Don't works, .NET Fiddle doesn't allocate enough memory

    }

    private static void Print(char start, char last)

    {

        foreach (var (n, c, sequence) in ABACABA.Calculate(Enumerable.Range(start, last - start + 1).Select(c => (char)c).ToArray()))

            Console.WriteLine($"{n:00}({c}): {sequence}");

    }

}

​

///<remarks>https://www.reddit.com/r/dailyprogrammer/comments/njxq95/20210524_challenge_391_easy_the_abacaba_sequence/gzaf51a</remarks>

public static  class ABACABA

{

    public static IEnumerable<(int n, char c, string sequence)> Calculate(params char[] alphabet)

    {

        var sb = new StringBuilder((int)Math.Pow(2, alphabet.Length) - 1);

        sb.Append(alphabet[0]);

        var n = 0;

        yield return (++n, alphabet[0], sb.ToString());

        foreach (var c in alphabet.Skip((1)))

        {

            Calculate(sb, c);

            yield return (++n, c, sb.ToString());

        }

    }

    private static void Calculate(StringBuilder sb, char middle)

        => sb.Append(middle).Append(sb, 0, sb.Length - 1);

}