Programming Interviews Exposed: Secrets to Landing Your Next Job, Second Edition

12.1. Tackling Brainteasers

One of the most important themes to keep in mind is that the solutions to brainteasers are almost never straightforward or obvious. Unlike the programming or technical parts of the interview, where you will sometimes be given simple problems just to see whether you know something, brainteasers always require thought and effort. This means that any solution that seems immediately obvious is probably incorrect or not the best solution. For example, suppose you're asked, "From the time you get on a ski lift to the time you get off, what proportion of the chairs do you pass?" Most people's immediate gut-level response is that you pass half of the chairs. This response is obvious and makes some sense. At any given time, half of the chairs are on each side of the lift, and you pass chairs only on the other side. It's also wrong — because both sides of the lift are moving, you pass all the other chairs.

This answer assumes you get on and off at the extreme ends of the lift. On most real ski lifts, you pass almost all the other chairs.

This property of brainteasers works most strongly to your advantage when you are faced with a problem that has only two possible answers (for example, any yes or no question). Whichever answer seems at first to be correct is probably wrong. Of course, it's probably not a good idea to say, "The answer must be yes because if it were no this would be a very simple problem and you wouldn't have bothered to ask it." You can, however, ...