So, if you try to enter a 7 when there is already a 7 in the same row, column, or 3x3 grid, it should give an error message instead
You can use a JOptionPane.showMessageDialog
This should use the Sudoku model class to check for this. Probably you should add a new method!
Help => Hint
Choosing this menu option should highlight any squares that can only contain one possible value
Victory!
If you have won it should let you know and end the game!
Only Legal values
Figure out some way to show only the legal values for a square. This can be a special keyboard
input (like if you click on an empty square and then click 'h' it will show you the legal
values somehow, either through a JOptionPane.showMessageDialog or even something like
a ListSelectionModel)
Load/Save games
Set up the load/save menu options so that they can load and save a game
To save a game, write the board to a text file. Use a JFileChooser to decide what file to write to
To load a game, use a JFileChooser to load a game from a text file
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