A free program for Android, by AlcamaSoft.

Why purchase Sudoko books when you can just download the Sudoku app? This app features four levels: easy, normal, hard, and very hard. If it’s your first time solving sudoku puzzles, it’s best to choose either the easy or normal difficulty level. You may also check out the in-app tutorial for techniques for solving sudoku puzzles. 

What’s best about this app is its unlimited redo and undo, making it less restrictive and more beginner-friendly.  Sudoku also includes a manual and automatic checking system so you can easily see your mistakes. Although the app contains microtransactions, you can still enjoy the Sudoku app as an F2P.