First of all, on the first day, let all prisoners count by the second, and let the prisoners on the first day know that they are the first. Then after he went out, he turned off the light and the second person who went out turned on the light again. Turn off the lights when the first person is drawn out for the second time. At this time, the next person who goes out will see the light turned off. Open it, it doesn't matter.

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By analogy, the following principles are followed:

1 Only the first person who goes out has the right to turn off the lights.

Everyone has only one chance to turn on the light.

In this way, when the first person turns off the light for the 99th time, it can be proved that it has been fully ventilated.