Sudoku

Fill every empty square with a digit from 1 to 9 so that no digit repeats in any row, any column or any of the nine 3×3 boxes. Every puzzle has exactly one answer, and you never have to guess: it can always be reasoned out.

Tap or click a square, then a digit on the pad. Tap the same digit again to take it back. Turn on ✎︎ Pencil to pencil in small candidates instead; placing a digit clears it from the notes it can see. Digits that break a rule turn red.

Keyboard: ←↑↓→ move, 1 to 9 fill, Shift+digit adds a note, Backspace/0 clears, N toggles notes, U (or Ctrl+Z) undoes, ? gives a hint, F2 is New puzzle, P pauses.

☞︎ Hint first points out any wrong digit or notes that are missing the answer; otherwise it fills in the next square you could have found and says how. Every level is graded by the hardest trick it needs: Easy and Medium only need singles, Hard needs pointing pairs and naked/hidden pairs, Expert needs X-Wings, Swordfish or XY-Wings.

Each level keeps its own game in progress, so switching levels never loses one. Best times count only for puzzles solved without hints.