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());
}