I highly recommend the Brainium sudoku app. https://brainium.com/games/sudoku/ https://brainium.com/games/sudoku/ It has selectable difficulty and a "hint" mo

I highly recommend the Brainium sudoku app. https://brainium.com/games/sudoku/ https://brainium.com/games/sudoku/ It has selectable difficulty and a "hint" mode that teaches you how to solve even the hardest sudokus without any backtracking search at all.