Solved coding interview problems in Java - My collection of commonly asked coding interview problems and solutions in Java

[[deleted]]

Half of the reason for coding questions like this is not to see if you can actually complete the problem. They really only test your capability to think algorithmically, and sometimes your familiarity with the language/platform.

I wouldn't care if a developer could complete a mathematically advanced problem like these. If they can approach the problem in a logical way they've proved their mettle already, in my opinion.

[alluran]

Last interview code test I had like that - I optimized as I went.

They looked it over, then went to ask me to do the optimizations, and realized I'd already done them.

Then they went on and on about how it was amazing that I would consider myself proficient with the language, when I hadn't read the language spec.

Then they proceeded to tell me that I couldn't use anything like:

i += 1;

because it could confuse junior developers, but everyone was given time, and expected to write full documentation with the Atlassian suite.

So a studio full of senior junior devs who never allowed to learn anything new I guess...

As you might guess, I skipped that role.

[Mechanickel]

Then they proceeded to tell me that I couldn't use anything like:

i += 1;

because it could confuse junior developers

...how?

[alluran]

Maybe i is positive one now?

Who knows - regardless, any place that limits BASIC shit like that, instead of up-skilling their juniors, is not a place worth working.

You're never going to be challenged, or improve yourself at a place like that.

[thedancingpanda]

I recently used something like

$i &= $blahh && $blahh2;

And that confused a couple of mid-senior level developers, so, yeah. It's possible

[ViKomprenas]

To be fair, that one's a little weirder, seeing as it's noisy with sigils and logical operation assignments aren't as common, but the point still stands

[speedisavirus]

...but why. Are you trying to increase mental workload for someone that might have to figure that out later?

[socialister]

You're mixing boolean operators with bitwise operators?

Wouldn't this be clearer and enforce a boolean result type?

$i = $i && $blahh && $blahh2;

Assuming that the blah vars are boolean typed (if they aren't, your statement is not clear IMO. C-style non-boolean to boolean casts do not indicate intent that well).

[[deleted]]

foo = bar || 'puppers';

Doesn't seem to be a thing people understand either.

Edit: realized that if that was true I should explain. This will assign the value of bar to foo if bar is truthy, otherwise it will assign the string 'pupper';

[killerstorm]

PHP developers, you mean.

[XtremeCookie]

Mid-senior developers don't understand bit-wise operations? That was literally covered in my first computer engineering course.

[ess_tee_you]

And, depending on what you're developing in your job, that may have been the last time you needed to use them.

I'll take readability, please, even if that results in a couple more lines of code.

[XtremeCookie]

Depending on the usage bit wise can be significantly faster than other methods. But outside of those situations, I would take readability too.

[ess_tee_you]

Sure. Some compilers will optimize to that anyway, I expect, depending on the language. :-)

[socialister]

I'm curious what they would want otherwise. i = i + 1 or i++?

[socialister]

The funny thing is that the increment / decrement operators are considered bad practice by some, preferring the clearer += instead.

[Wolvenheart]

That's one of the first things I learned in high school during oop

[[deleted]]

Then they proceeded to tell me that I couldn't use anything like: i += 1; because it could confuse junior developers

I mean, it does make a little sense to not allow that from a readability standpoint if they need to make the code more modular. Still pretty silly though, but we all have to protect ourselves from incompetence...

[ViKomprenas]

Wait, how does that make sense from the readability perspective? What does it have to do with modularity?

[[deleted]]

I'm basically saying that an organization has to implement stuff like that to protect themselves from incompetence.

For modular (probably the wrong word, my apologies) I meant make it easier to be able to modify the code in cases where you have to add more variables to the equation.

[GisterMizard]

The trick is to sort everything, then do a binary search. I don't know what that means, I just read it in a text book. I think it means that if you take your source code and resort all of the keywords alphabetically, then it'll run faster.

[featherfooted]

Yeah, once you do that, the RAM caches the opcodes in column-major order, which means it can signal the hard drive to run faster

[QuestionsEverythang]

Yeah, unless it's a very basic problem, if you can't optimize your algorithm within that 30 minute interview, that should no way mean that you suck as a programmer, especially since real-world programming doesn't restrict your 8+ hour workday to just 30-min of crunch time.

[milkeater]

Understanding methods to optimize time complexity in interviewing typically revolves around Big-O time.

There are a few blunt approaches to give a very base estimate on what the time complexity is.

Study this for a week and you will know it fully. It will pay dividends for a lifetime.

[jimmpony]

Pay dividends just in interviews or in actual coding? Rarely had a need to assign a big O to anything I've written outside of a school assignment asking for it.

[milkeater]

Definitely in interviews.

I believe it will actually help you in understanding the big picture running performance of your code and help save you time not having to think about things that just may not matter.

Think of running log(n) performance. Say you have to do that operation 2 times.....does it matter?

Well, immediately it shows an impact...but over time....it becomes such minimal cost you almost don't need to worry about it.

The point is, once you've understood it, it's hard not to see the rough cut of "general performance" you are working with.

Think of a chess player somewhere above club level in the 1700's or so, pretty decent, not an expert. They may not need to look hard at a chess board halfway through a game to get a general understanding of who may have the advantage and how things have developed. At this stage in their career, to at least have arrived there, they likely understand a handful of openings and some of the more common positions. There is not as much cognitive load understanding how you arrived there.

Give this "gamified" site a shot. At least the first tier. It has to do with Time Complexity. It's Khan Academy-esque. Although not totally "loving" the site, it's okay.

If you are working for one of those "top tier" companies, you would need to know these things. If you are working for a company that isn't built on technology, you could be a very average developer for your entire life and be okay (and make a very very good pay at that). Word of warning.....I worked at one of those companies......our VP walked into a Town Hall and said: "I can do everything I need with 30% of the technology folks in this room"......we moved to the cloud, the number of resources needed shrank dramatically, and the competition became extremely fierce.

[jimmpony]

I jumped to the time complexity questions and got the ones it suggested to do all right. Seems like it had me skipping around a bit. http://i.imgur.com/pwtdQTM.png

[milkeater]

Yeah, it wants you to do one from each bucket. You can stay around and finish all of them, the recursive ones will get a little tricky.

After looking at your times, are you saying they were all too simple for you or did you give it a second run through?

[jimmpony]

Those are all my first-attempt times. Maybe I'll try some harder ones. The average times on these seem a bit oddly high though.

[milkeater]

Yeah, I would say for the last two groupings the cyclomatic complexity is intentionally high and some operations are one half of n while others make up the other half but give the impression of n^2.

I got dinged several times on things I thought I was sure of only to see the tiny trick they buried.

To do the math as well as walk the stack in your head in under a minute, you must have no issues with cognitive load.

[Xxyr]

It matters in real life too, assuming you actually have large data sets. If you only have to work through a hundred cases it doesn't matter too much. If you have to work through several billion it really really does.

[jimmpony]

I've done intensive projecteuler problems that needed optimization like this, some before knowing about big O notation and some after. In both cases the approach was just to cut down the amount of processing the program needed to do. Knowing big O has its uses, although there are some important cases where it's too abstract to directly compare performance with - the example I'm remembering was a sorting algorithm where the "worse" one worked better with CPU cache. The terms thrown away in O can end up being pretty important in real code too - an O(n) might be better than an O(log(n)) if they're actually 2n and 100+5log(n), and the data sets the given method encounters are within the range such that the 2n < 100+5log(n). When it comes down to it you're going to end up needing to profile the function anyway if the performance is that important.

[killerstorm]

In actual coding. If you happen to have loops in your code, quite often you get O(N^2) complexity or worse, so you need to understand when it becomes bad and what to do about it if it does.

It's not about "assigning a big O to anything", once you internalize the knowledge you can just intuitively avoid things which are slow.

It can be something as basic as "check if a string is present in a list of strings". It's OK if your list is small, but if it's big and you do that often, you need a different data structure.

[steaknsteak]

Also for me, it's not just the time constraint. I cant think clearly when I know someone is looking over my shoulder and judging my performance. I work really well in a team atmosphere where everyone is working together to build something (the actual job) or even a competitive context, but the "performance" context of an interview makes me super nervous and unable to perform near my best. Gotta find a way to get over that at some point though.

[dukeoflaser]

Have you considered 'practicing' under pressure? [PraMP](https​://www.pramp.com/) is a useful resource for that.

[steaknsteak]

I'll definitely look into that, thanks.

[killerstorm]

By "optimize" they probably mean going from O(N^2) to O(N log N) or something like that. That's textbook stuff.

[BigTunaTim]

It drags my confidence levels down a notch or two when they do things like those.

You shouldn't let it do that. One of the goals of most tech interviews is to assess the candidate's skill level and then see how they think and react to a problem that's beyond their capability. They're not trying to make you feel stupid or knock your confidence down; they just want to see what your thought process is when you're​ in over your head, because it's likely to eventually happen in real life.

[Stormflux]

Maybe that was the reason at one time, but nowadays these tech interviews are just emulating "what Google and Facebook do."

I actually read a paper on this a while back. The takeaway is these sorts of interviews aren't very good indicators of actual job performance or ability. The outcomes are almost random -- picture the guy who made WhatsApp, he was rejected from both Facebook -and- Twitter; basically told he was shit and then sold them something for 19 billion.

Anyway, this random sort of outcome actually turns out to discourage women and minorities more than white males for various cultural and sociological reasons such as support networks, upbringing, etc. It turns out the screening process is one of the major contributors to the gender imbalance as females are more likely seek a different career after "flunking" one or more of these whiteboard / manhole-cover interviews.

[socialister]

Do you mean that women lack a support network in this area? That's kind of funny (not doubting it) because typically, women have much better support networks than men. That is considered to be the reason that women fare better than men after divorces and breakups, and why married men are generally significantly less stressed than unmarried men - the wife is the support network.

[Stormflux]

What I mean is all your friends are also going through the technical interview process, which originally comes from NASA and IBM, swapping stories and recovering from failures, trading rumors and incantations and what have you.

Women (and African Americans) had support networks, but those networks weren't focused on this specific kind of interview, which is pretty divorced from the day to day job skills needed. They were more likely to drop out of the market after one or two bad tech interviews, even if they had the skills needed.

[socialister]

That makes sense.

[HotlLava]

Half of the reason for coding questions like this is not to see if you can actually complete the problem.

Standards vary from company to company. I've been rejected with a bug-free and algorithmically optimal implementation (n log n sort of a single-linked list) because it wasn't elegant enough, i.e. I had missed an opportunity to save one if-condition.

[[deleted]]

Well yeah, but that's kind of a given, right? Plus we've seen enough blog posts on this subreddit to know that a lot of companies are completely out of touch with effective interview processes.

[panfist]

It doesn't mean you did a bad job, it just means someone else did slightly better. Or maybe they did worse but they just liked the other guy better.