alt Hacker NewsLiterally the second I read "it's a data stream" I knew the answer was going to be reservoir sampling.
RS is really interesting to me. many people you talk to can realize you can compute the mean of a data stream (https://www.geeksforgeeks.org/web-tech/expression-for-mean-a...) without knowing the exact formulation. And it's not far from that to think of a sampling strategy to decide if a new sample should go into a fixed-size reservoir. (for all of these, I know specific hints that will usually help people get to the next step).
The only reason I know RS is because it was in the google3 monorepo and I was looking for interesting codes to use and found it. There was an associated Sharding class, LexicographicRangeSharding (https://www.mongodb.com/docs/manual/core/ranged-sharding/) which you could use to find near-optimal split points in sorted string tables so your mappers didn't end up with hotspots. If you had shown me Algorithm R in a stats class, I don't think I would have appreciated it at all, but seeing the code implementation and a useful example made it click.
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>"it's a data stream" I knew the answer was going to be reservoir sampling.
But it's only an approximate percentile. Unless the interviewer mentions that an approximate solution is OK, you would be stuck. (And it's not fair to ask the candidate to ask whether an approximate solution is ok given that almost every problem has an easy "approximate" solution which is not explicitly not what they're looking for).
I'm pretty confident I would've been able to come up to the solution in better circumstances, maybe even without hints.
But it was clear in this case that the interviewer just took a question from the company's bank of questions and was alt-tabbed for half the interview, I have felt the energy early and I was also half-checked out.
I'm aware I'm saying this post factum, but I had a very fun first interview with that company and matched well with the first interviewer so my expectations were high, and then I got hit by the big tech style interview when it was an early stage startup.