using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static IEnumerable<int[]> GrayCodes(int length, int radix) {

      if (length < 0)

        throw new ArgumentOutOfRangeException(nameof(length));

      if (radix < 0)

        throw new ArgumentOutOfRangeException(nameof(radix));

​

      if (0 == length || 0 == radix)

        yield break;

​

      int[] signs = Enumerable.Repeat(1, length).ToArray();

      int[] current = new int[length];

​

      for (bool keep = true; keep; ) {

        yield return current.ToArray();

​

        keep = false;

​

        for (int i = current.Length - 1; i >= 0; --i) {

          int d = current[i] + signs[i];

​

          if (d >= 0 && d < radix) {

            current[i] = d;

​

            for (int j = i + 1; j < signs.Length; ++j)

              signs[j] = -signs[j];

            keep = true;

​

            break;

          }

        }

      }

    }

    public static void Main()

    {

        int length = 3; 

        int digits = 3;

        var report = string.Join(Environment.NewLine, GrayCodes(length, digits)

          .Select(line => string.Join(" ", line)));

        Console.WriteLine(report);

        Console.WriteLine();

        var flatten = string.Join(" ", GrayCodes(length, digits).SelectMany(g => g));

        Console.Write(flatten);

    }

}