Sudoku

Three difficulties according to how many numbers are removed:
a) easy: 25~34;
b) medium:35~44;
c) hard:45~55.
2. Hint the fitting numbers for row, column and block.
3. Save the unfinished puzzle.
4. Load the saved puzzle.
5. Clear the filled number.
6. Restart the same puzzle.
7. Offline game.