7 ms·

Facebook Engineering Puzzles

89 points by kachnuv_ocasek 16y ago

ajays 16y ago
Facebook seems to be following the same trajectory that Google did. I remember ~10 years ago Google used to put out such puzzles; if you solved them, your chances of getting hired at El Goog went up significantly. In another, say, 5 years, expect to hear about Facebook giving out massive retention bonuses to top engineers who are defecting en masse to a hot new startup... :-D
- wh-uws 16y ago
  Its the circle of (extraordinarily successful startup) life.
kqr2 16y ago
Here are some challenges from Quora, the Q&A site: http://www.quora.com/challenges http://www.quora.com/challenges
michaelchisari 16y ago
I've never understood the use of puzzles and tests for hiring. I'm notoriously bad at them, but I consider myself to be a very solid programmer, with 13 years professional web development experience, and a major open source project under my belt. I think one of the reasons I don't do well on these is because my brain doesn't seem to work well for hypotheticals. I need real world use cases or else it's exceedingly difficult for me to work out a solution for a problem that is too abstract. Maybe I'm unique amongst other programmers with my level of experience. It's possible that other programmers love doing puzzles like this, but I tend to get discouraged if I'm looking at a company, and they use tests and puzzles to gauge my capabilities. Especially (and I wouldn't accuse Facebook of this, but it happens much too often), if those tests and puzzles are substitutes for looking at the massive amounts of code I've written.
- endtime 16y ago
  I think the attitude of a lot of companies, especially the ones hiring fresh grads, is that the ability to think is more important than the ability to code (the latter of which can be taught far more easily than the former). I don't mean to imply that you are unable to think, of course - the puzzles are surely an imperfect heuristic - but that's my understanding of their aim.
- deadc0de 16y ago
  The Facebook puzzles are pretty easy. That being said, I hate puzzles too. I think it's pretty degrading to ask a well-established researcher/engineer to do that kind of stuff. On interviews I typically ask people to give a talk about their previous projects in detail. A lot of nuances arise during these talks that allow you to quite easily judge the level of the candidate.
  - hammerdr 16y ago
    Why is it degrading? I would think it would be perfectly acceptable for companies to ensure that their new hires (experienced or not) pass their basic heuristics. This comes in the form of tests, resume checks, social compatibility, etc. (I ask this from the perspective of a young, inexperienced developer.. so excuse the thoughtlessness!)
    - deadc0de 16y ago
      I don't know... I would personally feel a bit weird asking, for example, an experienced compiler developer with a CS PhD to reverse a linked list, or words in a string, or something like that.
      - soamv 16y ago
        So get them to solve something more difficult than reversing a linked list. A good programmer will enjoy a good puzzle, especially if it is new to them.
- prostoalex 16y ago
blhack 16y ago
Uh, maybe I'm just failing the first puzzle, but how do you submit these?
- prostoalex 16y ago
  The instructions are below the list of puzzles - "Follow all of the following directions or your submission may be considered incorrect." It's automated grading, so it's nitpicky about the submission format, etc.
  - Periodic 16y ago
    You have to be very careful about spaces and returns on some of them as well. Fortunately they tend to have short and precise output specifications. I do wish their grading script could give a little more indication of what failed, as I'm not sure it will even tell you the difference between a build error and a test failure. If it does, great. I struggled with that a year ago when I first worked on them.
- siddhant 16y ago
  Submissions should be sent via email to the address: {0xFACEB00C>>2 in decimal}@fb.com (Its right there, below the list of all puzzles.)
  - blhack 16y ago
    Ah, missed that, thank you. The email address is also listed on the fan page.
- ot 16y ago
  I hacked up a simple script to automate submissions (https://gist.github.com/757221 https://gist.github.com/757221). It handles a single file per puzzle, so an executable or an archive.
  - wmil 16y ago
    Has this been working for you? I haven't received any response. I'm not sure if I'm doing something wrong, or if their system is down.
palish 16y ago
Bah! No Lua :(
siddhant 16y ago
Is it considered a deal breaker if interviewees are unable to solve such puzzles during interviews?
- alexgartrell 16y ago
  The puzzles posed in interviews at Facebook (and at Google) have a lot less B.S. It's more geared toward data structures/algorithms. Failing to solve these puzzles is a deal breaker. here's a kind of medium example from Fog Creek: given a list of elements in lexicographical order (i.e. ['a', 'b', 'c', 'd']), find the nth permutation I'll add that they'll give you a certain amount of guidance if you require it, but beyond a certain point they're just helping you to make it less awkward.
kingkilr 16y ago
Meh, perhaps I've grown crotchety, but I'm not jumping through hoops for you (theoretical employer), sorry. I can solve puzzles just fine, but I'd rather spend the holidays implementing new PostgreSQL driver.
p01nd3xt3r 16y ago
I contacted the like a little guys and they sent me back 2 puzzles before they would even speak to me. The first one was dead simple and took about 14 seconds to write the code and another 7 for it to run and give me the answer. "A 7 digit number consists of 7 distinct digits (from 0..9 ofcourse); The product of the first 3 digits = product of central 3 digits = product of last 3 digits; Find the middle digit." "Middle Number: 2" "Set: 1892463" "Common Products: 72" The next question I did not understand; I just figured I was not smart enough because I have no college and I am just a hacker that makes shit not a "software eng.". I sent it to a friend of mine that teaches Econ. at a top college and he said he had no idea what they were even asking. (see question below) Given a list of N line segments on the X axis: (x1[i], x2[i]) where i = 1 to N; And then a list of M queries. Each query inputs one x-coordinate and you need to output the number of line segments that overlap this point. Assume M & N are very large. (So O(M*N) is really bad.) I asked for clarification and got none so I guess they figured i was just too stupid to respond to which is cool but it made me wonder if when people talk about programmers that cant program do they mean people that cant answer these types of questions? I spent 5 or 10 min or it and gave up. But even though I didn't answer this I can code like a MF and if this question had been a programming challenge I think I could have done it. My point is that companes should really think before sending out stuff like that to potential developers because they might pass up on some really good talent. I am not saying that I am very talented but I do know how to code and build cool products. I made digest.io and it uses clojure, ML, MapReduce to learn about your diet and help manage digestive conditions and it has paying customers that are happy. I made that and it works well so im not a rookie but I still have no idea what that 2nd question is asking. BTW: Even though i hate these puzzles the FB puzzles are simple and I have always been able to complete them quickly.
- btilly 16y ago
  They probably didn't answer because if you need clarification on the question, you have no hope of producing the answer. And incidentally it IS a programming challenge. (Which is why the econ prof had no clue.) You have a collection of N intervals. And a collection of M points (which they call queries). For a given interval and point, that point can be in the interval or not. For each point you are supposed to find out how many intervals it is inside of. And you're supposed to make the code fairly efficient. From the way you've written it, the intervals look like open intervals. So x1[i] is not in the i'th interval. That is a minor but significant detail, and I expect their test data to check it. The naive solution is to write a function that takes an interval and a point, and says whether that point is in that interval. Then you just loop over all points and intervals. This works, but the number of times you run that comparison is N * M. Which, if N and M are large, is going to be painfully slow. In fact there is a notation to discuss how slow, and that notation would say O(N * M). They don't want any variant on this solution. The solution they are probably looking for is O((N + M) * log(N)). One solution is to try to first find and put into sorted form all of the intervals where the answer is going to be constant across the interval, and then do a binary search for each point to find what interval it falls into (and therefore what the answer is for that point). My guess as to their thinking is that anyone who can't figure out something like this won't have any clue when they cause scalability problems. This matters for larger sites. If you don't understand any of what I just said, that's a hole in your background. To fix it go off and learn about little-o, big-O, and some basic algorithms. A short list of algorithms to learn could include quicksort, merge sort, hashing, binary search and how a BTree works. That combination will solve most problems pretty easily.
cletus 16y ago
I really like these puzzles. After you get through the first few trivial ones. The submission process is a little frustrating in that you have no idea if your code failed because it's wrong or because it simply took too long. Taking too long is a real problem with a few of the puzzles (eg breathalyzer). If you read the comment threads there are some astoundingly fast solutions out there. My C breathalyzer solution was pretty quick but I was running it on an Ubuntu VM running inside Windows 7 and that did impact performance (particularly I/O). I've been meaning to run it on a real Ubuntu setup but haven't gotten around to it. Anyway it all became moot as I ended up getting a job at Google but I did find it an interesting exercise.
pmorici 16y ago
I'm confused, what exactly do they mean by this bit, "If you compressed your solution, any Makefile/build.xml files (and the executable file itself) need to be uncompressed and/or built into the same root directory as the compressed archive itself or your submission will result in a build error." Are they saying that the archive shouldn't have any directories in it period or that it should all be in one directory?
- frou_dh 16y ago
  Sounds like neither. A prebuilt binary (or the makefile and resultant binary) should be at the top level. You could have other files in subdirectories if you liked.
deleted 16y ago
[deleted]
EGreg 16y ago
Here is why the puzzles can be very effective, especially for a company in no hurry to hire: 1) The people who submit them are most likely interested in your company, took the time to solve the problem, and have a mind that works well. That already saves a lot of time screening people you wouldn't want for those top positions. 2) It attracts other smart people because you wouldn't refer puzzles to stupid people -- notice how we posted it on Hacker News. On a similar note, if you google for Node.js like I did, you'll find a sponsored result from Meebo offering you to apply to their "secret jobs!" -- because not just anyone searches for Node.js, but rather people who are interested in the new technology, which they are probably using or looking into. 3) Psychology -- people feel like they want something they worked towards. They committed time to facebook, so they will take it more seriously, and when someone from facebook talks to them, and asks them about their background AFTER solving the puzzle, they will feel a sense of accomplishment and engage back. 4) Notice how many of these puzzles have specified the exact outputs. They can screen the initial applicants with an automated test suite to see how well they did. Cheapest first round candidate evaluation ever :) 5) The puzzles are hopefully related to the kinds of architectural and social challenges they actually have to solve at facebook, so this prepares the candidate for that kind of thinking. If the candidate doesn't like thinking in this way, perhaps it's better to find out as early as possible, and before even SPEAKING to the candidate is pretty early! If anything I would definitely set this kind of thing up for the top (architectural, etc.) development positions at facebook. They were recently named the best place to work at in 2010, so they can afford to do this.
admp 16y ago
There is also Facebook Hackercup on the way: http://www.facebook.com/hackercup http://www.facebook.com/hackercup
- admp 16y ago
  Why have I been downvoted? Facebook suggests to practice with the puzzles linked by OP before registering for Hackercup, so these are very well connected.

ajays 16y ago

Facebook seems to be following the same trajectory that Google did. I remember ~10 years ago Google used to put out such puzzles; if you solved them, your chances of getting hired at El Goog went up significantly. In another, say, 5 years, expect to hear about Facebook giving out massive retention bonuses to top engineers who are defecting en masse to a hot new startup... :-D

wh-uws 16y ago
Its the circle of (extraordinarily successful startup) life.

kqr2 16y ago

Here are some challenges from Quora, the Q&A site: http://www.quora.com/challenges http://www.quora.com/challenges

michaelchisari 16y ago

I've never understood the use of puzzles and tests for hiring. I'm notoriously bad at them, but I consider myself to be a very solid programmer, with 13 years professional web development experience, and a major open source project under my belt. I think one of the reasons I don't do well on these is because my brain doesn't seem to work well for hypotheticals. I need real world use cases or else it's exceedingly difficult for me to work out a solution for a problem that is too abstract. Maybe I'm unique amongst other programmers with my level of experience. It's possible that other programmers love doing puzzles like this, but I tend to get discouraged if I'm looking at a company, and they use tests and puzzles to gauge my capabilities. Especially (and I wouldn't accuse Facebook of this, but it happens much too often), if those tests and puzzles are substitutes for looking at the massive amounts of code I've written.

endtime 16y ago
I think the attitude of a lot of companies, especially the ones hiring fresh grads, is that the ability to think is more important than the ability to code (the latter of which can be taught far more easily than the former). I don't mean to imply that you are unable to think, of course - the puzzles are surely an imperfect heuristic - but that's my understanding of their aim.
deadc0de 16y ago
The Facebook puzzles are pretty easy. That being said, I hate puzzles too. I think it's pretty degrading to ask a well-established researcher/engineer to do that kind of stuff. On interviews I typically ask people to give a talk about their previous projects in detail. A lot of nuances arise during these talks that allow you to quite easily judge the level of the candidate.
- hammerdr 16y ago
  Why is it degrading? I would think it would be perfectly acceptable for companies to ensure that their new hires (experienced or not) pass their basic heuristics. This comes in the form of tests, resume checks, social compatibility, etc. (I ask this from the perspective of a young, inexperienced developer.. so excuse the thoughtlessness!)
  - deadc0de 16y ago
    I don't know... I would personally feel a bit weird asking, for example, an experienced compiler developer with a CS PhD to reverse a linked list, or words in a string, or something like that.
    - soamv 16y ago
      So get them to solve something more difficult than reversing a linked list. A good programmer will enjoy a good puzzle, especially if it is new to them.
prostoalex 16y ago

blhack 16y ago

Uh, maybe I'm just failing the first puzzle, but how do you submit these?

prostoalex 16y ago
The instructions are below the list of puzzles - "Follow all of the following directions or your submission may be considered incorrect." It's automated grading, so it's nitpicky about the submission format, etc.
- Periodic 16y ago
  You have to be very careful about spaces and returns on some of them as well. Fortunately they tend to have short and precise output specifications. I do wish their grading script could give a little more indication of what failed, as I'm not sure it will even tell you the difference between a build error and a test failure. If it does, great. I struggled with that a year ago when I first worked on them.
siddhant 16y ago
Submissions should be sent via email to the address: {0xFACEB00C>>2 in decimal}@fb.com (Its right there, below the list of all puzzles.)
- blhack 16y ago
  Ah, missed that, thank you. The email address is also listed on the fan page.
ot 16y ago
I hacked up a simple script to automate submissions (https://gist.github.com/757221 https://gist.github.com/757221). It handles a single file per puzzle, so an executable or an archive.
- wmil 16y ago
  Has this been working for you? I haven't received any response. I'm not sure if I'm doing something wrong, or if their system is down.

palish 16y ago

Bah! No Lua :(

siddhant 16y ago

Is it considered a deal breaker if interviewees are unable to solve such puzzles during interviews?

alexgartrell 16y ago
The puzzles posed in interviews at Facebook (and at Google) have a lot less B.S. It's more geared toward data structures/algorithms. Failing to solve these puzzles is a deal breaker. here's a kind of medium example from Fog Creek: given a list of elements in lexicographical order (i.e. ['a', 'b', 'c', 'd']), find the nth permutation I'll add that they'll give you a certain amount of guidance if you require it, but beyond a certain point they're just helping you to make it less awkward.

kingkilr 16y ago

Meh, perhaps I've grown crotchety, but I'm not jumping through hoops for you (theoretical employer), sorry. I can solve puzzles just fine, but I'd rather spend the holidays implementing new PostgreSQL driver.

p01nd3xt3r 16y ago

I contacted the like a little guys and they sent me back 2 puzzles before they would even speak to me. The first one was dead simple and took about 14 seconds to write the code and another 7 for it to run and give me the answer. "A 7 digit number consists of 7 distinct digits (from 0..9 ofcourse); The product of the first 3 digits = product of central 3 digits = product of last 3 digits; Find the middle digit." "Middle Number: 2" "Set: 1892463" "Common Products: 72" The next question I did not understand; I just figured I was not smart enough because I have no college and I am just a hacker that makes shit not a "software eng.". I sent it to a friend of mine that teaches Econ. at a top college and he said he had no idea what they were even asking. (see question below) Given a list of N line segments on the X axis: (x1[i], x2[i]) where i = 1 to N; And then a list of M queries. Each query inputs one x-coordinate and you need to output the number of line segments that overlap this point. Assume M & N are very large. (So O(M*N) is really bad.) I asked for clarification and got none so I guess they figured i was just too stupid to respond to which is cool but it made me wonder if when people talk about programmers that cant program do they mean people that cant answer these types of questions? I spent 5 or 10 min or it and gave up. But even though I didn't answer this I can code like a MF and if this question had been a programming challenge I think I could have done it. My point is that companes should really think before sending out stuff like that to potential developers because they might pass up on some really good talent. I am not saying that I am very talented but I do know how to code and build cool products. I made digest.io and it uses clojure, ML, MapReduce to learn about your diet and help manage digestive conditions and it has paying customers that are happy. I made that and it works well so im not a rookie but I still have no idea what that 2nd question is asking. BTW: Even though i hate these puzzles the FB puzzles are simple and I have always been able to complete them quickly.

btilly 16y ago
They probably didn't answer because if you need clarification on the question, you have no hope of producing the answer. And incidentally it IS a programming challenge. (Which is why the econ prof had no clue.) You have a collection of N intervals. And a collection of M points (which they call queries). For a given interval and point, that point can be in the interval or not. For each point you are supposed to find out how many intervals it is inside of. And you're supposed to make the code fairly efficient. From the way you've written it, the intervals look like open intervals. So x1[i] is not in the i'th interval. That is a minor but significant detail, and I expect their test data to check it. The naive solution is to write a function that takes an interval and a point, and says whether that point is in that interval. Then you just loop over all points and intervals. This works, but the number of times you run that comparison is N * M. Which, if N and M are large, is going to be painfully slow. In fact there is a notation to discuss how slow, and that notation would say O(N * M). They don't want any variant on this solution. The solution they are probably looking for is O((N + M) * log(N)). One solution is to try to first find and put into sorted form all of the intervals where the answer is going to be constant across the interval, and then do a binary search for each point to find what interval it falls into (and therefore what the answer is for that point). My guess as to their thinking is that anyone who can't figure out something like this won't have any clue when they cause scalability problems. This matters for larger sites. If you don't understand any of what I just said, that's a hole in your background. To fix it go off and learn about little-o, big-O, and some basic algorithms. A short list of algorithms to learn could include quicksort, merge sort, hashing, binary search and how a BTree works. That combination will solve most problems pretty easily.

cletus 16y ago

I really like these puzzles. After you get through the first few trivial ones. The submission process is a little frustrating in that you have no idea if your code failed because it's wrong or because it simply took too long. Taking too long is a real problem with a few of the puzzles (eg breathalyzer). If you read the comment threads there are some astoundingly fast solutions out there. My C breathalyzer solution was pretty quick but I was running it on an Ubuntu VM running inside Windows 7 and that did impact performance (particularly I/O). I've been meaning to run it on a real Ubuntu setup but haven't gotten around to it. Anyway it all became moot as I ended up getting a job at Google but I did find it an interesting exercise.

pmorici 16y ago

I'm confused, what exactly do they mean by this bit, "If you compressed your solution, any Makefile/build.xml files (and the executable file itself) need to be uncompressed and/or built into the same root directory as the compressed archive itself or your submission will result in a build error." Are they saying that the archive shouldn't have any directories in it period or that it should all be in one directory?

frou_dh 16y ago
Sounds like neither. A prebuilt binary (or the makefile and resultant binary) should be at the top level. You could have other files in subdirectories if you liked.

deleted 16y ago

[deleted]

EGreg 16y ago

Here is why the puzzles can be very effective, especially for a company in no hurry to hire: 1) The people who submit them are most likely interested in your company, took the time to solve the problem, and have a mind that works well. That already saves a lot of time screening people you wouldn't want for those top positions. 2) It attracts other smart people because you wouldn't refer puzzles to stupid people -- notice how we posted it on Hacker News. On a similar note, if you google for Node.js like I did, you'll find a sponsored result from Meebo offering you to apply to their "secret jobs!" -- because not just anyone searches for Node.js, but rather people who are interested in the new technology, which they are probably using or looking into. 3) Psychology -- people feel like they want something they worked towards. They committed time to facebook, so they will take it more seriously, and when someone from facebook talks to them, and asks them about their background AFTER solving the puzzle, they will feel a sense of accomplishment and engage back. 4) Notice how many of these puzzles have specified the exact outputs. They can screen the initial applicants with an automated test suite to see how well they did. Cheapest first round candidate evaluation ever :) 5) The puzzles are hopefully related to the kinds of architectural and social challenges they actually have to solve at facebook, so this prepares the candidate for that kind of thinking. If the candidate doesn't like thinking in this way, perhaps it's better to find out as early as possible, and before even SPEAKING to the candidate is pretty early! If anything I would definitely set this kind of thing up for the top (architectural, etc.) development positions at facebook. They were recently named the best place to work at in 2010, so they can afford to do this.

admp 16y ago

There is also Facebook Hackercup on the way: http://www.facebook.com/hackercup http://www.facebook.com/hackercup

admp 16y ago
Why have I been downvoted? Facebook suggests to practice with the puzzles linked by OP before registering for Hackercup, so these are very well connected.