Sudoku

Every puzzle is generated on the fly and guaranteed to have exactly one solution. Difficulty is judged by the solving techniques a puzzle actually requires — not by how many clues it happens to show.

Desktop — arrow keys or WASD move the selection, 1–9 fill, Backspace / Delete erase, N toggles notes, Z undoes, H gives a hint.

Touch — tap a cell to select, then use the number pad. Notes toggles pencil marks; Erase clears a value or the notes in the selected cell.

Hints come from the same logical solver used to grade the puzzle. They explain the next deductive step; they never simply fill in an answer.

Check briefly marks wrong entries; it never reveals the answer.