A free app for Android, by Metajoy.

Exercise your brain and solve challenging number puzzles in Sudoku – Classic Puzzles. There are four difficulty levels available that will test your solving skills. You can choose from easy, medium, hard, and expert. When the level is too difficult for you, you can use notes and hints to help you get through it.

You do not have to pay for anything because the game is entirely free to play. You can also play at your own pace since it has no time limits.

Sudoku – Classic Puzzles offers a smooth, clean, and user-friendly interface. It does not require internet connection so you can play anytime and anywhere you want.