Fill the grid so every row, column and 3×3 box holds the digits 1 to 9 once each.
Notes lets you pencil small candidate digits into a square. They clear themselves when a digit you place rules them out.
Hint fills the selected square, or a random empty one. Undo steps back. Wrong digits show in red and count as mistakes, but never end the game.
Every puzzle is freshly generated and has exactly one solution.