[acedio]

That's a better solution in general IMO, but the author's approach can guarantee that you'll never go over N calls in a sliding window rather than fixed windows. I don't believe that's possible with the timestamp + count solution. Gotta bring up both solutions and ask the interviewer what they want :)

[ALittleLight]

What's the problem with a sliding window and the timestamp + count solution? lastTime is the timestamp of the last call. newTime is the timestamp of the incoming call. If newTime - lastTime > 60 seconds then you're good to proceed, set count to 1 change lastTime to newTime and go on. Otherwise, check whether count is less than n and proceed accordingly (incrementing count if so). This accomplishes the sliding window and rounding down to the last whole minute handles the fixed window - right?