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LightsOut is a puzzle where you are given a grid of cells, or lights, with some dark and others light. You must turn them all off by clicking on the cells. Each click toggles that cell and each of its immediate neighbors.
Play our LightsOut now or read on for a method to solve these puzzles.
All valid LightsOut puzzles can be solved without clicking on any cell twice. This is because clikcing on a cell twice is the same as not clicking it at all.
The easiest way to solve LightsOut puzzles is to use a method called Chase The Lights. Starting with the second row, click on every cell that has a light on in the row above it. This will turn off all the lights in that row. Continue with each successive row until the only remaining lights are in the final row.
Now that you only have lights on in the final row, use the lookup table below and find the pattern of lights. This will tell you which lights to click in the top row. When you chase the lights this time, the bottom row will end up dark, just like the rest of the puzzle.
| Bottom Row Lights | Top Row Clicks |
|---|---|
| ..*** | ...+. |
| .*.*. | .+..+ |
| .**.* | +.... |
| *...* | ...++ |
| *.**. | ....+ |
| **.** | ..+.. |
| ***.. | .+... |
For a 9x9 puzzle, you will either end up with a dark bottom row, or *.*.*.*.* as your pattern. If you have the latter, click the top left cell, and chase it to turn out all the lights.
Obviously this method will often result in more moves than necessary, as you will sometimes end up clicking a cell twice as you chase the lights two times. To figure out the fewest possible moves, take the click pattern for the top row, and start with that on the original puzzle. This time you will only need to chase the lights once.
This still might not result in the fewest possible moves, as chasing the following top row click patterns result in no change to a 5x5 puzzle:
.+++. +.+.+ ++.++
So applying one of these top row click patterns before you start the puzzle may result in fewer clicks.
For a 9x9 puzzle, there are even more combinations of top row clicks that don't change the puzzle win chased to the bottom. Clicking any even numbered cell in the top row, or clicking any two odd numbered cells will do this, as will any combination of such clicks. So, while a 9x9 puzzle is fairly easy to solve, it can be quite difficult to solve it in the fewest possible clicks.