Sudoku

Goal of the game

Fill the 9 × 9 grid with the digits 1 to 9 so that every row, every column and each of the nine 3 × 3 boxes contains each digit exactly once. The digits printed at the start cannot be changed.

Controls

• Tap a cell, then a digit on the pad (or type it on the keyboard).
• Notes (N key): write small pencil candidates in a cell.
• Auto notes: fills in every possible candidate in one go.
• Erase (Backspace, Delete or 0) removes a digit or the notes.
• Keyboard arrows: move the selection. Ctrl + Z: undo.

Hints

The Hint button first spots any mistakes you may have made, then shows the next logical deduction: the cells involved light up and an explanation in English describes the technique. Tap Hint a second time (or Apply) to carry it out.

Levels

• Easy and Medium: naked and hidden singles.
• Hard: pointing, box-line reduction, pairs and triples.
• Expert: X-Wing, Swordfish, XY-Wing, XYZ-Wing.
• Diabolical: XY-Chains.

About the AI

Every puzzle is generated from a random solution, then hollowed out cell by cell as long as an exhaustive solver confirms that the solution remains unique. A second solver, which reasons like a human with sixteen techniques, grades the puzzle: its level matches the most advanced technique that is indispensable. The same solver provides the hints and the Solve button, which completes the grid before your eyes without ever guessing. Generation takes place in a separate thread, in a few milliseconds.