Leetcode image

Having switched jobs a few times over the last few years, I’ve done a a lot of software engineering interviews. In my most recent job search, for instance, I did around eight phone screens followed by six on-sites.

The most stressful part of any interview for me, and for many others as well, is the “technical coding” questions that inevitably get asked; most companies require you to solve at least one during a phone screen, and then another two to four once you get onsite.

Over the last few job searches, I’ve developed a strategy that has significantly improved my performance on these questions, to the point that I’m now rarely phased by them. In this post, I’d like to share what I’ve learned.

Practice, practice, practice

The key to doing an excellent job in a coding-oriented software interview is very simple- you really, really need to practice beforehand. The interesting thing about practice is that it’s more effective, more necessary, and more profitable than one would intuitively think.

Why it’s more effective than you think

Software is such a broad field and there are so many technologies and applications that could come up in an interview! Sure, practice can help, but how can it really make that much of a difference?

The reality, however, is that only a small subset of these are appropriate for an interview. In particular, companies generally restrict their questions to those that:

  1. Can be explained in under 5 minutes
  2. Can be solved by a good candidate, starting from scratch, in under 30 minutes
  3. Don’t require specialized, domain-specific knowledge (e.g., networking, databases, cryptography, graphical user interfaces, etc.)
  4. Don’t require external data, documentation, hardware, or software libraries / code
  5. Touch on intro-undergraduate-level algorithms and data structures including sorting, searching, dynamic programming, lists, hashmaps, etc.

These restrictions significantly reduce the pool of possible questions. Even if you get a question that isn’t exactly like one you covered in your practice, chances are that it has some similarities to one you have seen, and you can use those to your advantage.

Why it’s more necessary than you think

Another misconception is that if you’re already doing a lot of hands-on software development as part of your day-to-day job, this replaces the need for practice. That would make sense if companies only asked practical questions, but the reality is that most don’t. In fact, the vast, vast majority of questions I’ve gotten over the years have little direct overlap with the work I’ve done in the past or, most likely, the work that I would do if hired by the interviewer’s company.

The reason for this gets back to the criteria listed above; day-to-day projects rarely, if ever, satisfy all of these. Most questions are completely synthetic and, thus, you need practice beyond your day-to-day work to really get a handle on them.

Why it’s more profitable than you think

What many people don’t realize is that interview performance isn’t just a binary, “hire or no hire” checkpoint on the way to a job offer. At many companies, even if you meet the hiring bar, your performance is used as an input in setting your starting level and compensation. Doing an excellent job on your coding interviews, as opposed to just a good one, may lead to a stronger initial offer and give you more negotiating leverage as you move through your job search.

Enter Leetcode

Leetcode is a site designed for practicing interview-style coding questions. It has a large pool of questions and an online code editor that allows you to write up and submit solutions to them.

Each submission is run against a set of question-specific test cases that verify both the correctness and efficiency of the solution. If you get stuck, you can get help by looking through official solution write-ups (available for the most popular questions) or reading the messages on each question’s discussion board.

There are a bunch of other sites in the general, “code interview prep” space. I’ve been less satisfied with the ones that I’ve looked at, however, because their questions feel less realistic to me; they’re often not at the right level of difficulty, or they’re simply too long to do in a 45 minute interview. These questions may be fun to work through, but if your goal is to prep for interviews, solving them may not be the most efficient use of time.

Note that I have no official relationship with Leetcode. I just like the site and want to tell others about it. :-)

Leetcode question difficulty

Leetcode classifies questions as either “easy”, “medium”, or “hard”. I’ve found that the sweet spot for actual interviews typically begins at the middle of the “medium” tier and runs into the bottom (i.e., easier) chunk of “hard”. “Easy” questions are fine to do as stepping stones to the “medium” tier, but you’re less likely to see these types of questions in an actual interview.

The hardest “hard” questions in Leetcode are annoying because they often require obscure algorithmic tricks to solve efficiently. I don’t worry about these questions too much because, in the unlikely event that I were to get a question in this tier, the interviewer would (hopefully) provide hints. They also don’t generalize as well as the easier questions, so studying them is not the most effective use of time.

Prep strategy

Once you’ve gotten your Leetcode account set up, it’s time to actually start preparing.

Language choice

Leetcode supports the languages that are most commonly used for interviews including C++, Java, Javascript, Python, and Ruby. You should obviously practice in the one(s) that you intend to use.

I would highly recommend choosing a single, high-level language for all of your interviews. I personally always use Python; Ruby is also a reasonable choice. C++, on the other hand, is probably not a great choice unless you happen to be a virtuoso in the language.

There are numerous advantages to choosing a high-level language like Python:

  1. Compact, less verbose code: Assuming a fixed typing or white boarding speed, this means that you can construct solutions faster.
  2. No IDE required: Many interviews are still done on whiteboards or in bare-bones text editors, so you can’t always depend on having an IDE to help out.
  3. Rich standard libraries: Python, for instance, provides binary search (in bisect), permutations (in itertools), heaps (in heapq), and many other things that are helpful for solving interview questions.

Of course, the company you’re interviewing at might use something else, like Java or C++. But, that’s generally ok. In fact, in my many years of interviewing I’ve never had an interviewer complain about my use of Python or felt that it ever counted against me. The most important thing is to deliver clean, correct code; provided you do this, the language you choose isn’t really an issue.

Working through the questions

When I’m ready to start, I pay for a premium Leetcode subscription. It’s a bit pricey (about $35 / month as of early 2019), but I find it worth the money because it removes restrictions in the free product including access to locked questions and the ability to sort questions by frequency. You can still use Leetcode if you don’t pay, but you’ll have to do some offline research to figure out which questions are the most important ones to focus on.

I then sort the questions in decreasing order of frequency (only available after paying for premium) and go through them roughly in order. I say “roughly” because I like to initially focus on the easier questions, and then, as I get more confidence, shift to the ones in the medium-to-hard range which, as discussed above, are more typical in actual interviews. Also, some questions are just slight variations of ones above them in the list, so I’ll skip over these if I already feel comfortable with them.

Once I’ve decided to do a question, I’ll work on it independently for about 20 minutes. If I can’t get a reasonable solution within this time, I’ll peek at the official solution for a few hints and then get back to work; this is similar to how it would work in an actual interview, with the difference being that in the real thing a person (i.e., your interviewer) would be offering these hints.

I stop working on the question when either I get all the test cases to pass or I hit the 45 minute mark. Whether I’ve gotten a working answer or not, I read through the solution to make sure that I have a correct understanding of the problem and the possible approaches to it. If I didn’t get a fully working solution within 45 minutes, I’ll go back and implement one of the approaches in the solution write-up, making sure that I fully understand what I’m writing down.

Solution efficiency

Many Leetcode questions can be solved in more than one way. Given a choice, I generally start with the simplest approach. Then, if this isn’t good enough, I’ll optimize.

In a real interview, this push to optimize would be done by the interviewer. In the Leetcode case, you’ll usually know you need to optimize because a test case will time out. In the rare event that a wildly suboptimal, naive solution passes all the test cases, you’ll notice this by seeing that your solution is in the far right tail of the execution time histogram that Leetcode shows at the end.

Consulting external sources

Most interviewers will allow you to look up general API docs or code snippets online if you ask. However, doing this can waste a lot of time and thus should be used sparingly in real interviews. It’s fine to do lots of lookups early in your preparation, but by the end all of the basic tasks in your language of choice (e.g., iterating over a hash map, reading a file, etc.) should be committed to memory.

How much studying is enough?

I usually feel ready to do real interviews after I’ve solved around 100 questions. Doing more than this has diminishing returns for me because the questions begin to look alike and my performance doesn’t significantly improve. It takes me at least 3 weeks to get through this many questions, assuming that I’m spending most of my weekends studying.

Ultimately, the goal isn’t to hit a specific number but rather to feel comfortable with arbitrary questions in the interview difficulty range (mid-medium to low-hard, as discussed above). You may get to this point after only a few dozen questions or, if you’re particularly rusty, it may take many more than 100. Likewise, the time required will also vary, from just a week or two to 6 months or more.

Leetcode gaps

Leetcode has good coverage for most types of coding questions, but I have noticed some gaps over the course of my recent job hunts. In particular, you may want to do some non-Leetcode preparation for the following:

  1. Interface design: Leetcode has to provide an interface so that it can run test cases against your solution. In a real interview, the interface might not be provided and, moreover, designing it might be a big part of the question.
  2. Writing tests: Leetcode provides its own test cases, so you don’t have to write any. In a real interview, you may be expected to test your solution, so you should feel comfortable doing this in your language of choice. Using something simple like assert statements is fine; there’s usually no need for complicated unit testing frameworks.
  3. Big-O analysis: Some interviewers like asking about time and space efficiency after you write up a solution; Leetcode doesn’t explicitly test this, although the official solutions write-ups usually include some efficiency analysis.
  4. Concurrency: Leetcode doesn’t provide much coverage for concurrency-related problems. You may want to brush up on concurrency primitives in your interview language, particularly if you don’t use these as part of your day-to-day work.
  5. Network and file system operations: Leetcode solutions run in a sandbox, and thus aren’t going to be making HTTP requests or reading files from the file system. These types of questions aren’t super common, but some companies do like them so you may want to brush up on how to do them in your chosen interview language.

During my most recent rounds of interviewing, there were some types of questions that came up again and again. If you’re on the market, you may want to spend some time making sure you’re extra comfortable with questions of these types.

Searching and/or exploring in a 2D grid

Interviewers love these questions because they hit on multiple topics at once: recursion, dynamic programming, 2D arrays/lists, spatial reasoning, etc. They also lend themselves nicely to follow-up challenges if you finish the question quickly and the interviewer wants to give you more.

See Leetcode 62 and 200 and for examples.

Evaluating arithmetic expressions

These questions are popular because they’re easy to explain and test, but can be challenging to implement. See Leetcode 227 for an example.

Designing and implementing key/value stores or caches

Caches are practical and easy to explain, but, depending on the desired performance tradeoffs, require a good understanding of certain data structures and algorithms. Leetcode 146 is a classic of this type.

Concluding thoughts

Preparing for coding questions makes these questions easier to solve and significantly increases your chances of doing well in job interviews. However, no strategy is perfect, and ultimately, no matter how smart or well-prepared you are, tech interviewing can still be a bit of a crapshoot. If you get rejected, use the experience to better understand your weak spots and further refine your preparation strategy.

Good luck!!


I’d like to thank John Mishanski for providing feedback on early drafts of this post.