Fill every row, column and 3×3 box with the numbers 1 to 9, each used once. Tap a square, then tap a number; empties it again. Notes lets you pencil in the numbers a square could hold. Hint tells you which rule the next step needs; a finish that used Hint counts as solved with help.
Keys: 1–9 enter a number, 0 or ⌫ clears, N notes, Z undo, Y redo (also with Ctrl or ⌘), arrows move.