5 ms·

I’ve had that question twice in google phone screen interviews, and iirc, the best answer has changed from the hash table thing to just use the hardware instruc

I’ve had that question twice in google phone screen interviews, and iirc, the best answer has changed from the hash table thing to just use the hardware instruction.

praptak 6y ago
I don't believe companies want to hire for knowing that popcnt exists. The interviewer should follow up with the hypothetical situation when either the language or the CPU doesn't have the instruction. Nobody should ask this anyway, this has become the fizzbuzz type of question.
- wiredfool 6y ago
  Yeah, it's a stupid question, which is why I was surprised that Google asked it to me twice, separated by a couple of years. What's funny is that 15 years later, I actually had a use for popcnt, put it together with something that seemed expensive at the time, and wound up with a C program that exhaustively searched a problem space in .12 sec. On a single core, in a vm, on a laptop. So much for me trying GPU programming on that one. (as in , feckit, there aren't that many possibilities, we'll just count them all)
- ericbarrett 6y ago
  Long before leetcode, I used to ask an "algorithm" question in interviews. I don't remember it exactly, but IIRC it was about determining quickly if a number was 2^n-1 for any n. (Don't shoot me, this was a decade ago.) There were three broad ways candidates answered it: 1) Hack out a for-loop-style bit count. This was good, because even though it wouldn't be optimized, it demonstrated they could understand the problem and at least conceptualize a solution. 2) Give the "leetcode" best-answer (it was some bitwise math trick). I'd then ask if they'd seen the problem before, and the answer was always yes. This was a mark in their favor—but no better than (1)—and also a signal I needed to ask them a follow-up question that actually made them think. 3) Code a bit, but not quite arrive at a solution. I'd then probe them about their thought process. Not an automatic fail, sometimes our brains just don't walk down the corridors we want them to at a given time, especially during an interview. (One candidate couldn't hold the marker steady because his hands were shaking too much! Poor guy.) 4) Give up and say they had no idea. Obviously the worst case for the interviewee. The purpose was not to check a candidate's recall and test-taking abilities, but rather to watch them reason through a novel problem, appropriately limited in scope for the interview timeframe. Case (2) served as a great short-circuit to block test-preppers lacking actual experience.

I’ve had that question twice in google phone screen interviews, and iirc, the best answer has changed from the hash table thing to just use the hardware instruction.

praptak 6y ago

I don't believe companies want to hire for knowing that popcnt exists. The interviewer should follow up with the hypothetical situation when either the language or the CPU doesn't have the instruction. Nobody should ask this anyway, this has become the fizzbuzz type of question.

wiredfool 6y ago
Yeah, it's a stupid question, which is why I was surprised that Google asked it to me twice, separated by a couple of years. What's funny is that 15 years later, I actually had a use for popcnt, put it together with something that seemed expensive at the time, and wound up with a C program that exhaustively searched a problem space in .12 sec. On a single core, in a vm, on a laptop. So much for me trying GPU programming on that one. (as in , feckit, there aren't that many possibilities, we'll just count them all)
ericbarrett 6y ago
Long before leetcode, I used to ask an "algorithm" question in interviews. I don't remember it exactly, but IIRC it was about determining quickly if a number was 2^n-1 for any n. (Don't shoot me, this was a decade ago.) There were three broad ways candidates answered it: 1) Hack out a for-loop-style bit count. This was good, because even though it wouldn't be optimized, it demonstrated they could understand the problem and at least conceptualize a solution. 2) Give the "leetcode" best-answer (it was some bitwise math trick). I'd then ask if they'd seen the problem before, and the answer was always yes. This was a mark in their favor—but no better than (1)—and also a signal I needed to ask them a follow-up question that actually made them think. 3) Code a bit, but not quite arrive at a solution. I'd then probe them about their thought process. Not an automatic fail, sometimes our brains just don't walk down the corridors we want them to at a given time, especially during an interview. (One candidate couldn't hold the marker steady because his hands were shaking too much! Poor guy.) 4) Give up and say they had no idea. Obviously the worst case for the interviewee. The purpose was not to check a candidate's recall and test-taking abilities, but rather to watch them reason through a novel problem, appropriately limited in scope for the interview timeframe. Case (2) served as a great short-circuit to block test-preppers lacking actual experience.