It gave me a test concerning finding the "equilibrium index" of a sequence. It said to assume that the sequence is very long. (The question is repeated here: http://forums.sun.com/thread.jspa?threadID=676596&start=...) This problem seems underspecified, because it doesn't state whether you can iterate repeatedly over the sequence in question. If you can, it's easy. If you can't I'm not sure how to do it.
I assumed that "sequence is very long" meant "the time (and space?) complexity of your solution matters". Iterating over the sequence x times doesn't change the time complexity, so long as x << n (i.e. it's O(n) whether x is 1, 2 or 3).
Unfortunately, revealing whether my assumption proved correct would give away how they assess the solution to this demo problem, which some might view as a spoiler.
EDIT: since people are posting actual solutions, I suppose mere analysis of the testing methodology is okay. My assumption was correct.
Does that ever seem like a good idea? The description suggests it may be a long list. That should give you a hint that you don't need to do this to solve the problem.
The best case possible is to iterate the list once - you sum the list from left to right holding cumulative answers and if the final sum is zero, that index is a valid answer.
The worst case of the best algorithm is to iterate twice - firstly as above, and then again in the opposite direction, comparing values from the first iteration.
