TODO for this assignment

Disallow entering illegal values

• 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