public override string ToString()
        {
            var stringBuilder = new StringBuilder();

            for (var i = 0; i < SudokuBoard?.GetLength(0); i++)
            {
                for (var j = 0; j < SudokuBoard?.GetLength(1); j++)
                {
                    if (SudokuBoard[i, j].HasValue)
                    {
                        stringBuilder.Append($"{SudokuBoard[i, j].Value}|");
                    }
                    else
                    {
                        stringBuilder.Append(" |");
                    }
                }
                stringBuilder.AppendLine();
            }

            return(stringBuilder.ToString());
        }