Show HN: Coding Interview Practice

https://www.interviewcake.com

by gameguy43 • 12 years ago

172 111 12 years ago

Yes! I got the first question right. Somebody should hire me as a programmer. :D

myblake• 12 years ago

Looks like the site has been improved considerably since I last checked it out, love the new content and context (especially the story about Alice in the about page).

VonKeebles• 12 years ago

First question is wrong, I could have just sold short. ;)

anorark2• 12 years ago

https://www.interviewcake.com/all-questions (link is commented out in the page source)

gameguy438• 12 years ago

Yeah, that page is there so people who have finished all questions can go back and review. You can basically get around the paywall using this link. I figure people who dig around to find a way around the paywall probably wouldn't have paid anyway :P

akbiggs2• 12 years ago

If I pay to get all the questions only to learn later that I could have accessed them all easily for free, I'm going to feel ripped off, even if it was only $5. Either make sure the questions are blocked off properly or let everyone have access to all of the questions. You can't have it both ways here.

anorark2• 12 years ago

I was actually not trying to find my way around the paywall :) I didn't know you had to pay to access all 14 questions.

suyash3• 12 years ago

thanks @anorark!

sciguy77• 12 years ago

The line, "this is not just another technical question database" is really effective. You anticipate the first possible criticism someone might have and dispel it. I'll definitely give this a whirl soon, great work.

suyash3• 12 years ago

But it kind of IS and the fact that the quality and quantity of questions is far inferior to some of the other books on interview prep I've looked at in the past.

adambard• 12 years ago

I'd say the step-by-step hint process is the secret sauce. It mimics an actual interview scenario (where a generous interviewer might be willing to help you along) while teaching by example some structured ways to break down the problem by walking through it step by step. Notably, most of the answers start with a brute-force solution and optimize from there, which (IMO) is an excellent way to approach these sorts of problems, especially in a stressful situation such as a real job interview.

elilux• 12 years ago

Interviewcake.com is awesome. At TechCareerHQ, we buy all our student memberships because of the quality of the questions and ability of students to focus on one question at a time. After using Interviewcake, our students are much more prepared for interviews.

codezero• 12 years ago

I love this idea and the way the problems are presented.

One suggestion (based on the one question I saw) is to also mention at least in passing, concerns with edge cases in the inputs. Python often lets you ignore these things but interviewers tend to be charmed by at least an attention to the possibility that your input is empty.

Also, it might be worthwhile to have some "homework" problems that don't have solutions that are variations of the main problem.

For example, the stock price problem is sometimes presented as changes in price rather than absolute price. This adds some subtle differences that might throw people off unless they've tried it.

I imagine similar variations exist for all the problems available.

xixixao• 12 years ago

Say at Microsoft interviewers will drill you on edge cases (but it really is common in most shops). That means all empty / invalid input as well as overflow, floating point errors etc. Some places care about reliability vs speed or elegance more than others.

stygiansonic4• 12 years ago

A bit of a nitpick, but the stock trading question should have stated up front that short-selling was not allowed or that the purchase order must precede the sell order. (I suspect someone with some familiarity with trading would have asked these questions in an actual interview for clarification)

Relying on a "hint" to figure out that you can only sell after you buy seems more like an additional constraint rather than a true hint.

mehrdada• 12 years ago

Indeed. Having had quite a bit of experience with ACM-type competitions, I believe it is super-important for technical coding questions not to leave room for misinterpretation (especially when the key point being tested relies on the accurate interpretation, like this case.) Being sloppy in the interview may lead a bad taste in the mouth of the best candidates, because they are the ones who will take the question most literally.

It's not an excuse to leave things like that vague so that a candidate would have to bring it up to you. If you want to ask open-ended questions, ask obviously open-ended questions; don't make an attempt to turn simple technical questions into open-ended ones.

stygiansonic4• 12 years ago

Agreed. I feel that if a question is going to be about a "real world" situation like trading stocks, it should reflect real world constraints that should be stated up front.

I also found it curious that the example listed the stock price at 1 AM. It would be far more of a real world example to only consider times during the normal trading hours.

A question phrased about a particular real-world domain that doesn't makes sense within that domain just doesn't seem fair to ask.

But again, total nitpick on my part. The actual format in which the questions were presented was generally done well, and a nice refresh from the typical "interview prep" sites.

gt384u• 12 years ago

I feel like completely specifying the problem does everyone in an actual engineering interview a disservice and isn't a good analogue to actual engineering problem-solving. That said, it might be acceptable or even encouraged for the purposes of this site.

In the case that you mention, if someone asked me "Can I short sell?" I'd be happy that they considered the order of events there and were trying to explore the problem. I'd certainly remember it in my interview feedback. My response as an interviewer would be more or less "That's an interesting thought. To simplify the problem, let's say we're risk averse and prefer a finite loss potential. Go ahead and just consider purchase before sale."

One of the biggest criteria I looked for from junior engineering candidates was that they didn't just jump into implementation and asked clarifying questions. That's hard to do with the sort of site this is so maybe in this case more complete specification is useful, but any way that a bit of ambiguity can be introduced means that you can have a meaningful conversation with an engineer about representation, the shape of the problem, the inputs, what the criteria for success are, what is truly necessary to solve a problem. I want working code in the end, but hearing the things a person does or does not consider about a problem is interesting and valuable to understand a potential teammate and how they think.

stygiansonic4• 12 years ago

I don't have a problem with a vague question and in fact a candidate asking clarifying questions is indeed a Good Thing.

However the way the question was phrased just makes it seem like the person who phrased it doesn't understand how trading works in real markets.

cpwright2• 12 years ago

My first thought was that just getting a min and a max was the right answer, mainly because I having something hard-to-borrow is the edge case; and that the general case is that you can short.

jimmaswell3• 12 years ago

why would I need a hint that I can't time-travel? why would I need to be familiar with trading to know I can't sell something I bought in the future?

stygiansonic4• 12 years ago

Short-selling does not involve time-travel.

coupdejarnac• 12 years ago

Indeed. Or this becomes an exercise in calling sort().

ipsin• 12 years ago

The problem involving a duplicate number from 1..n... The sum 1+2+...+n is just n(n+1)/2. Something worth knowing for this sort of "trick" problem.

jw20133• 12 years ago

What if the summing overflow? Why not just XOR all the numbers in the set with 1...n (assuming the question is there is only one duplicate in integers in range [1,n]).

gameguy438• 12 years ago

Both great notes. Will workshop the question to include mention of constant-time ways to get the sum. Thanks!

chaoxu3• 12 years ago

I feel what you asked is a really nice follow up question, as tricks to not get a overflow could be highly non-trivial.

One can do addition mod n. As long as 2n doesn't overflow, you are good.

You can further improve it by not using any number larger than n. Say we want to sum a+b mod n, wlog assume a<b. If a+b<=n, we are good. If not, then b>n-a. Note a+b mod n = a+(n-a)+(b-(n-a)) mod n = (b-(n-a)) mod n. Now the calculation doesn't overflow.

Here is something to show avoid overflow can be quite difficult... http://cstheory.stackexchange.com/questions/19591/avoiding-o...

li2• 12 years ago

Alice got in facebook just attended coding boot camp and did some algorithm practices. Why does facebook recruit a non experienced like Alice but turns off lots of talented (with a top school cs degree)/with decent working experiences? I am curious.

smtddr• 12 years ago

>Why does facebook recruit a non experienced like Alice but turns off lots of talented (with a top school cs degree)/with decent working experiences?

I don't know what you're considering "talented", but Google seems to believe that your school/GPA has almost nothing to do with how well you'll perform on the job.

NOTE: Let's not confuse this with how likely you are to become another Zuckerberg. Starting a company is completely different from working at one. I don't think Zuckerberg could work at Google. He'd be bored out of his mind and/or argue with everyone, then quit in less than a year.

https://www.linkedin.com/today/post/article/20130620142512-3...

"GPAs don’t predict anything about who is going to be a successful employee. “One of the things we’ve seen from all our data crunching is that G.P.A.’s are worthless as a criteria for hiring, and test scores are worthless — no correlation at all except for brand-new college grads, where there’s a slight correlation,” Bock said. “Google famously used to ask everyone for a transcript and G.P.A.’s and test scores, but we don’t anymore, unless you’re just a few years out of school. We found that they don’t predict anything. What’s interesting is the proportion of people without any college education at Google has increased over time as well. So we have teams where you have 14 percent of the team made up of people who’ve never gone to college.”

monkeynotes• 12 years ago

Could use some syntax highlighting to make the code examples more easily read.

vladimirralev• 12 years ago

It's getting a bit ridiculous how much effort and talent is now dedicated to preparing candidates for interviews.

elwell• 12 years ago

Doesn't matter what the actual need is; the point is that there is lot of demand for people who want to prepare for interviews.

7Figures2Commas• 12 years ago

If only they knew that their newfound interviewing skills will in most cases land them...a job doing maintenance on a pedestrian CRUD app.

It's a sad state of affairs when the most intellectually challenging part of your job is the coding test you had to pass to get it.

_vn5r2• 12 years ago

I'm from Central America. I went to SF and had a few interviews over a month. I failed in all those 40-minutes-solve-as-many-as-possible-fizzbuzz-problems because I wasn't fast enough.

That said, I've been working as a web developer for 11 years now, bulding CRM's, CMS', Crawlers, automation tools, and all kind of web based stuff companies needed.

Also, I never went to college, and have been working before finishing high school (15 years old).

So, while I lack a lot of computer science education, I consider myself competent at developing solutions, and have progressed with coding practices.

But because of the US interviewing style, which I had never gone throug before, I am not employable there. Interview Cake may be just what I need : )

robotron• 12 years ago

Look beyond SF.

danielweber3• 12 years ago

It's tough out there. Most companies suck at finding candidates, and most candidates suck at finding employers. Very very few are doing the Matasano route of work samples and then measuring to make sure that this is actually working out.

In the meantime, as candidates we do what we can.

pmiller23• 12 years ago

Can you name any other companies who are using the work sample method?

bulatb2• 12 years ago

The fact that people need to study for their interviews is evidence that interviews are broken.

suyash3• 12 years ago

Any one has a link to all the solutions? It's annoying to get to those via hints all the time.

Zaheer• 12 years ago

I used to have a site like this but only show one hint. Love the concept of having multiple levels of hints though!

www.InterTechTion.com

Edit: Link to archive of all questions I sent out: http://www.intertechtion.com/answers/

gameguy438• 12 years ago

Sorry the site's a little slow guys. I'm up to 100 dynos on Heroku but things are still a bit sluggish at times. Looks like I need to move more stuff over to the CDN? Thanks for the load testing!

pauleastlund• 12 years ago

Good execution on a simple premise. Another terrific resource that I've pointed a lot of friends to is Project Euler -- a little heavy on the math but a lot of interesting questions that are great prep for the Google (or Facebook, or Twitter, etc.) gauntlet.

elyrly• 12 years ago

Parker (creator) does a wonderful job of laying out the different techniques to preform well on technical interviews. The format is easily understood and flow of questions is gradual enough to traverse.

incision• 12 years ago

I like this a lot.

Went ahead and signed up as some of what's there is probably bit over my head. Looking forward to seeing it grow.

I'd love to see even more discussion on each solution. Ideally something like an interactive "The Practice of Programming" [0].

0: http://www.amazon.com/dp/020161586X/

WoodenChair2• 12 years ago

Hey gameguy43, Great site! Can you tell us about the tech you used to build it? I see Heroku... Is that Bootstrap too? What language/framework/database are you using? Thanks!

gameguy438• 12 years ago

Hey there! Yep, bootstrap and angularjs in the front, Django in the back. Cloudfront CDN to handle the static assets (I've /really/ fallen in love with their product--so easy to set up). And of course, Stripe for payments.

gameguy438• 12 years ago

Love to get some thoughts from people on the pricing. Today the first 3 questions are free, and it's $5 for access to the other 11. How would people feel about $10? $20? What if there were 30 questions, sorted into different topics?

mondayRain2• 12 years ago

I feel as though to top something like "Cracking the Coding Interview", you're going to have to get close to the breadth they cover for the price they charge. You mention they 'charge' $25 which for sure is more than your $5; however, they've got loads and loads of questions for each data structure.

Currently, you're selling each question for about 35 cents. Although that's super cheap per question, $5 for 11 questions somehow doesn't seem like a great deal. However, I think people will be willing to pay the same for a larger database of questions e.g. I would be happy to pay $25 for 80 questions).

kimagure• 12 years ago

I paid the $5 because the sales pitch is so convincing -- "for the price of a burrito, you can get a job" (and so I did pay for this and I did get a new job!)

edit: back when I used this in Feb, I constantly got logged out and had to log back (was using G+ logins). Did this get fixed?

yblu• 12 years ago

Only 14 questions in total? And they cover "trees, graphs, depth-first search, greedy algorithms, brute force algorithms, dynamic programming, memoization, divide and conquer, amortized analysis, and more?"

cpwright2• 12 years ago

I'd expect it to provide about the same value as a book, and be browseable; not something that is purely linear one after the other (e.g., I could skip to dynamic programming, graph algorithms, or whatever topics there happen to be). I think that a basic interview puzzle book should be in the range of $25-$40, and depending on that price point be from 150 to 300 pages, with roughly 50-100 problems.

Though, admittedly, I wouldn't pay for this at the moment because I'm happily employed. I did spend about $100 on books to brush up when I was interviewing in late 2011. I found actually the best practice for interviewing was going on interviews (I ended up interviewing at around dozen companies over the span of a month after I was laid off).

debt• 12 years ago

I wonder if they'll eventually allow anyone to go to this site too be interviewed, live, by an actual engineer. For example, a prospect just shows up and jumps right into a technical interview with an interviewer from an interested company. It'd be good practice from the perspective of the engineer and act as potential job applicant from the perspective of the interviewer. Totally non-committal. I would imagine multiple prospects doing the interviews at the same time with the same interviewer.

Instead of scheduling an interview, you have prospective companies posting their interview schedules. For instance, "Hi, I'm from Google. I'll be hosting a technical interview at 330 PST. If you'd like to potentially work for Google, go to this link and answer my question in the time allotted. I'll be in the room to answer any questions." Of course, there's no whiteboarding involved but it might act as a pretty decent vetting process.

jasonhanley• 12 years ago

This is such a huge indication of the horribly sad state of interviewing and hiring programmers.

The current process results in hiring people that are awesome at interview quiz questions, but can't sit down and do the actual day-to-day work.

jarjoura2• 12 years ago

It goes without saying that solving algorithm questions are only a part of the interview process. Candidates are also judged on culture fit and design skills.

derwiki2• 12 years ago

'Culture fit' -- the wild card that lets you reject any candidate.

d23• 12 years ago

Before actively looking for my second job, I really had no idea how to interview folks effectively. After going through the process many times over the last month, I've come to the conclusion that there were essentially 3 good ways to evaluate me:

1) Give me a take home problem.

Granted, a couple of the companies that gave me these problems clearly didn't know how to interpret what I gave back to them. One of the comments I got back actually offered as a criticism that my code looked "too sparse." But if you actually have the ability to make sound judgments, this seems to me to be the ideal way to judge a candidate.

No one is coding over the phone (ever) or with someone watching over their shoulder even most of the time, so if you reject someone just because they can't perform under such pressure, you're going to have a lot of false negatives.

2) Ask a breadth of questions.

One of the other interviews which I excelled at was a series of 20 questions that asked me about things ranging from what happens when a browser makes a request to a server, to what a CDN was, to what the difference between UDP and TCP was. This was only a small part of the range of what was asked, but the fact that I was able to speak with confidence on most of the issues showed that I had a strong foundation of knowledge. They also asked my confidence in each area before asking me the questions, which I imagine helped them gauge whether I was down to earth enough to even recognize the things I didn't know.

3) Ask a depth question.

Drill down deep into a specific problem that the interviewee seems comfortable enough in. For this one, it could be entirely theoretical, such as how one might build a particular system to solve a problem. The key is that you want to dig into why they make any particular choice along the way.

You should look for someone willing to say "I don't know about X, I'd probably look for the reference to learn if there's a data structure to solve Y" if you end up getting too deep. That being said, not all personalities will understand that this is okay during an interview, as by the same token, a selection of interviewers won't understand that this is a sign of strength, not weakness.

chrisfarms• 12 years ago

The step by step expansion are really great, and I'm sure this will help many.

I think I prefer the learning by doing approach like talentbuddy[1] personally.

[1] http://www.talentbuddy.co/

mondayRain2• 12 years ago

Wait, so just to clarify - there are only 14 questions?

jimmaswell3• 12 years ago

Spoilers in its URLs? I'm on the search engine one and the URL says "compress-url-list"

I guess I was a bit off-base, I was thinking about using a database.

Well cool, this trie thing it suggests is pretty much what I was thinking, and something I'd thought about before without knowing what it was called.

danielweber3• 12 years ago

Holy cat snacks, this page uses a lot of third-party JavaScript.

jarjoura2• 12 years ago

I'm up-voting this just for your use of "Holy cat snacks," BWAHAHA.

wyclif• 12 years ago

I haven't tried it yet, but will later tonight. I just dropped by to say that this is a great problem to solve. It addresses an emerging market.

RA_Fisher4• 12 years ago

I have zero desire to play these silly games. Sorry, I'm much more than a 40 minute probability puzzle! The only way to win is stop playing. Give me a problem to take home, and think about. Or, how about asking me about my skills (and having a conversation about a piece of code). How about pair-programming? TDD abilities count for nothing, whaaaa?

These types of tests do little more than reinforce stale ideas and turn away unconventional well-rounded candidates. Used to play the game, now I win because I don't play.

RA_Fisher4• 12 years ago

Like one of their examples, "I have an array stockPricesYesterday where the keys are the number of minutes into the day (starting with midnight) and the values are the price of Apple stock at that time" My response would just be, "you generally shouldn't store data in keys". Their response, "Okay, just solve the problem." No real discussion.

WoodenChair2• 12 years ago

I'd give you brownie points but still expect you to answer the question. These questions are not THAT hard - they are definitely fair game and help screen out the people who only know how to use APIs and not how to think about things at a lower level.

But I agree an array would be a more efficient data structure for this :)

joe_the_user• 12 years ago

Absolutely,

Interview questions should hinge on, like, the experience and competence that I already have from having worked a substantial period of time as a professional computer programmer. Not on how cleverly I do on BS.

And I actually generally do very well on this crap but I still hate being impelled to take part in this kind of circus. In fact, it's annoying that I jump to being a top candidate based on this stuff and only then does an employer come up with some mundane requirement that should have been the initial qualifier/disqualifier.

022• 12 years ago

It is amazing to me to see computer science fundamentals hand-waved away as 'crap' and 'BS'.

gameguy438• 12 years ago

I think there's some truth to this. Companies should weigh past work more heavily, and let candidates do real work with the team as part of the interview process, IMO.

That being said, I think these whiteboard problems can be useful when you think of them as "trying out what it feels like to brainstorm with the candidate about how to solve a problem." That's how I think of them. There are plenty of people I just can't get on the same page with when trying to whiteboard together (different communication styles/speeds/whatever), and it's helpful to know that ahead of time.

bulatb2• 12 years ago

The problem is these interviews aren't brainstorms. They're exams, where you don't even know the range of topics in advance. No matter much interviewers claim it's just a conversation, the reality is they have standard questions with specific optimal solutions, chosen for their ease of grading rather than their relevance or what they actually reveal.

When no one knows the answer and the object of the meeting is to find it, it might really be a brainstorm. When the interviewer knows the answer and expects the candidate to get it, where a real discussion isn't possible because suggesting things that aren't The Answer (or asking stupid questions) could significantly hurt your chances, it's a test.

nerfhammer• 12 years ago

Think of it this way:

Take 20 developers. If you give them a test with 200 difficult problems they would all score about the same. Instead, give them 4 difficult problems in a limited time frame. Now some of them will look much smarter than others. Only hire these candidates. Now you can believe you are part of a very selective group.

xsmasher2• 12 years ago

"Having a conversation about a piece of code" sounds good, it proves you can read code and not just write it. Everything else you mention should have been taken care of on your resume or you wouldn't be in the interview, BUT plenty of people can talk a good game.

As an employer, do you really want to find out AFTER hiring that the candidate is incapable of understanding a problem based on an oral or written description; incapable of capturing requirements, asking the right questions, and forming an attack plan without thinking about it overnight?

Testing someone's ability to follow along and contribute in real-time is part of the test, IMO. It does penalize "deep thinkers" and still waters, though.

I've had candidates email me a supremely perfect solution the next day, and that's a positive data point too.

alashley• 12 years ago

Subscriber here, I felt like I needed interview preparation to be able to go through this interview preparation. I think it would be helpful to list some resources for those less well-versed with these types of problems.

Will you be adding more questions in the future?

I do like the concept of the site!

gameguy438• 12 years ago

Hey there,

The problems are definitely a bit more advanced. I do try to make sure that any "vocab words" (data structures, algorithm types) can be clicked on for a deeper explanation. But if you haven't seen big-o before, for example, you'll be in the dark on a few things right now. I have some more "bottom-up" curriculum that I've developed from teaching at boot camps around San Francisco, and I'm still working on figuring out the best way to present it on the site.

For Big-o specifically (the thing that looks like "O(n)"), I really like these resources: http://stackoverflow.com/questions/487258/plain-english-expl... http://www.perlmonks.org/?node_id=227909 http://www.daveperrett.com/articles/2010/12/07/comp-sci-101-...

Best of luck!

Pitarou• 12 years ago

For the indexing the web question (what data structure should an under-resourced web-crawler use to keep track of visited links?) wouldn't a Bloom filter work better than a trie?

scotth• 12 years ago

They mention it, actually.

Jach• 12 years ago

I noticed on the 'Validate string with open/close is well formed' question an edge case that's not handled. If the input has a trailing closer char with an unmatched opener, an IndexError will be raised because the stack will be empty when you try to pop from it. (Unless your particular Stack class just returns None or something? But then you'll get a KeyError when trying to access that key in the dict.)

huangc10• 12 years ago

you know what would be great? having the answer in multiple languages :) just an idea. good luck, nice site, I like it.

dxbydt2• 12 years ago

This is a very good list - I was asked a bunch of these & thankfully I was able to solve them & land the job. It's significantly harder to solve these if you require immutability (which they did in my case, since we work with Scala)

For example, the stock price problem (https://www.interviewcake.com/question/stock-price ), if you insist the solution has no mutables, I really don't know how a Java/Python/C guy would approach it. In Scala, you'd do

       import math._
       val s = List.fill[Int](100)((random*1000).toInt)
       s./:((s.head,0)){(a,b)=>(min(b,a._1),max(a._2,b-min(b,a._1)))}._2

bcoates• 12 years ago

Can't you always mechanically convert an iteration into a fold like that?

NoodleIncident• 12 years ago

I can't really understand your version, I'm not familiar with scala. Is this somewhat equivalent? I tried to use Clojure's loop/recur but I'm too much of a noob. :(

    def max_profit(stocks):
        def r(stocks, profit, lowest):
            return r(stocks[1:],
                     max([profit, stocks[0] - lowest]),
                     min([lowest, stocks[0]]))
        return r(stocks[1:], 0, stocks[0])

dxbydt2• 12 years ago

I think so. You've basically reimplemented a fold, but clojure stdlib has a built-in left fold. See line 84 https://github.com/clojure/clojure/blob/master/src/clj/cloju...

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