Yet Another Sudoku

• Choose your level of difficulty (The Extreme board starts with 1,000,000,000,000,000,000,000,000,000 possible permutations. That's 10^26) and begin a new game.
• Touch a cell to display a number picker and choose the number to enter in that cell.
• Long-press a cell to make notes of possible values for this cell and use those notes to narrow down the options. You can also mark the cell as a pivot point when you want to try one of two possibilities and remember where you might need to back up to.
• Use the undo button to back up to the very beginning if you need to.
• Use the menu or action bar to ask for a hint. You can also tell the app to warn you when a move violates the rules of the board, preventing you from costly mistakes.
• The timer is there for convenience and for your own interest -- See how fast you can solve each difficulty level.