Fill the grid so every row, column and 3×3 box holds the digits 1 to 9 exactly once.
Look for the row, column or box that already has the most digits. If eight of nine cells are filled, the ninth is forced — no thinking required. Work those first; each one you fill makes the next easier. Only when nothing is forced any more do you look at a single empty cell and ask which digits its row, column and box still allow.
The trap is guessing. A well-made sudoku never needs it: at every point there is at least one cell you can determine with certainty. If you find yourself trying a digit to see what happens, you have missed something simpler somewhere else.
Each sudoku in Puzelo is checked by a solver written separately from the generator, using a different method. A puzzle is only published if that independent solver finds exactly one solution — so you never need to guess, and the answer we print is never wrong.
More puzzle types: Nonogram · Binary Puzzle · Kakuro · Futoshiki