static void Main(string[] args)
{
int?[,] initialBoard = new int?[, ] {
{ null, null, null, 2, null, null, null, null, 1 },
{ null, null, 3, 8, null, null, null, 9, null },
{ 7, null, 4, null, 9, 5, 8, null, null },
{ 2, 8, null, null, null, null, null, null, 5 },
{ null, null, null, null, null, null, null, null, null },
{ 6, null, null, null, null, null, null, 7, 3 },
{ null, null, 2, 7, 5, null, 6, null, 9 },
{ null, 7, null, null, null, 6, 4, null, null },
{ 5, null, null, null, null, 9, null, null, null }
};
SudokuBoard board = new SudokuBoard(initialBoard);
bool solved = SudokuSolver.Solve(board);
Console.WriteLine(board.ToString());
Console.WriteLine($"{(solved ? "SOLVED" : "UNSOLVABLE")}");
}