DNA Sequences

            int magic = int.Parse(Console.ReadLine());

            for (int i = 1; i <= 4; i++)
            {
                for (int j = 1; j <= 4; j++)
                {
                    for (int k = 1; k <= 4; k++)
                    {
                        char s1 = '\0', s2 = '\0', s3 = '\0';

                        switch (i)
                        { case 1: s1 = 'A'; break; case 2: s1 = 'C'; break; case 3: s1 = 'G'; break; case 4: s1 = 'T'; break; }

                        switch (j)
                        { case 1: s2 = 'A'; break; case 2: s2 = 'C'; break; case 3: s2 = 'G'; break; case 4: s2 = 'T'; break; }

                        switch (k)
                        { case 1: s3 = 'A'; break; case 2: s3 = 'C'; break; case 3: s3 = 'G'; break; case 4: s3 = 'T'; break; }


                        char ends = (i + j + k >= magic) ? 'O' : 'X';

                        Console.Write($"{ends}{s1}{s2}{s3}{ends} ");
                    }
                    Console.WriteLine();
                }
            }