Sudoku classic.

Classic sudoku game for your iPhone and iPad.
Features:
- Four difficulty levels (easy, normal, hard, very hard).
- Tutorial with several techniques (from beginner to advanced) for solving the game.
- Unlimited undo and redo.
- Game saving.
- Notes.
- Auto noting empty squares.
- Auto updating notes.
- Square coloring with two colors for advanced techniques.
- Error checking (manual and automatic).
- Share the game with a friend via whatsApp, email, etc...