static void print_rows()
        {
            int[] nums = new int[] { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
            Random rand = new Random();
         
            for (int i = 0; i < nums.Length; i++)
            {
                for (int a = 0; a < nums.Length; a++)
                {
                    Console.Write(nums[rand.Next(9)]);
                    if (nums[a] == 9)
                    {
                        Console.WriteLine("");
                    }
                    if (nums[a] == 3 || nums[a] == 6)
                    {
                        Console.Write(" ");
                    }
                    if (i == 2 && nums[a] == 9 || i == 5 && nums[a] == 9 || i == 8 && nums[a] == 9)
                    {
                        Console.WriteLine("");
                    }
                }
            }
        }