Note: Read this problem carefully, because it's completely different from the original.

As before, 100 prisoners are put into solitary cells, and there's a room with a light bulb. (No prisoner can see the light bulb from his or her own cell.) Every night, the warden picks a prisoner at random, and that prisoner goes to the living room. While there, the prisoner can toggle the bulb if he or she wishes. but this time, the prisoner needs to assert that he knows, which prisoner was in the living room before him. If the assertion is false, all 100 prisoners will be shot. However, if it is indeed true, all prisoners are set free. Thus, the assertion should only be made if the prisoner is 100% certain of its validity.

The prisoners are allowed to get together one night, to discuss a plan.

But, the prisoners know that after that night, when they will go back to their solitary cells. the warden will choose one prisoner secretly (and this time, not randomly) and will kill him.

What plan should they agree on, so that eventually, someone will make a correct assertion?

We start of with some bloody smart prisoners who have the memories of an elephant.   We take two of those prisoners.   For the first 100 days, that prisoner is the one who gets to turn on the light.   For the next 100 days, the second one gets to turn on the light.   These two prisoners keep alternating every 100 days.   Hopefully this gives each of them plenty of opportunity to be chosen at random.   The absolute first prisoner turns off the light if it is on.If one of the two gets killed, the other is still there.

The number 100 is chosen merely to give a good chance that they will be randomly chosen during their turn.   Over time, one of the two will be chosen eventually.

Posted by Brandon on 2007-03-19 17:42:03