Fill every empty cell with a hollow square or a filled one, so that each row, each column and each of the two long diagonals — the lines drawn corner to corner — holds as many of one as of the other.

Never three alike one after another — not in a row, not in a column, and not along either of those two diagonals.

Tap a cell for a filled square, tap again for a hollow one, once more to clear it. The squares the board came with stay put: tap one and they all give a shake, to show which they are.

Shown in red: three of the same kind one after another, and squares a row, column or diagonal has too many of.

Learn the solving techniques