Hard

Lights #730 · 7×7

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7×7board
21lights on
19presses to solve
49cells

Lights #730 is a 7×7 board at hard level. It starts with 21 of its 49 lights on, and the one solution needs 19 presses.

Row 4 is the brightest, with 5 lights on. If you chase from the top, that is where most of the work happens.

One corner light is on. A corner light has only two neighbours, so it is changed by just three presses: itself and the two lights next to it.

With 19 presses in the solution, chasing from the top down and fixing the first row from what is left at the bottom is safer than trying to see the answer at once.

Hard 7×7 boards need a full chase, often twice. Keep count: pressing the same light twice cancels out.