[smithbits]

How big was the game board? It doesn't seem like running life backwards would be an obvious thing. If I have a 3 by 3 grid with the center turned on I think there are 8 choose 3 or 56 possible previous boards that would lead to that. I guess you're just looking for any possible previous board not a specific one but it seems like any decent sized board would lead to a seriously large problem space in a hurry. I also dislike puzzle questions in interviews and never use them when I'm hiring people.

[edit] Here's a cool page at wolframscience about testing for reversibility http://www.wolframscience.com/nksonline/page-1017a-text?firs...