Saturday, December 17, 2016

On reversing btrees in job interviews

Hiring new people is hard. No-one has managed to find a perfect way to separate poorly performing job applicants from the good ones. The method used in most big companies nowadays is the coding interview. In it the applicant is given some task, such as reversing a btree, and they are asked to write the code to do that on a whiteboard without a computer.

At regular intervals this brings up Internet snarkiness.

The point these comments make are basically sound, but they miss the bigger point which is this:

When you are asked to reverse a btree on a whiteboard, you are not being evaluated based on whether you can reverse a btree on a whiteboard.

Before we look into what you are evaluated on, some full disclosure may be in order. I have interviewed for Google a few times but each time they have chosen not to hire me. I have not interviewed people with this method nor have I received any training in doing so. Thus I don't know what aspects of interview performance are actually being evaluated, and presumably different companies have different requirements. These are just estimates which may be completely wrong.

Things tested before writing code

Getting an onsite interview is a challenge in itself. You have to pass multiple phone screens and programming tests just to get there. This test methodology is not foolproof. Every now and then (rarely, but still) a person manages to slip through the tests even if they would be unqualified for the job. Starting simple is an easy way to make sure that the applicant really is up to the challenge.

The second big test is one of attitude. While we all like to think that programming is about having fun solving hard problems with modern functional languages, creating world altering projects with ponies, rainbows and kittens, reality is more mundane. Over 80% of all costs in a software project are incurred during maintenance. This is usually nonglamorous and monotonous work. Even new projects have a ton of “grunt work” that just needs to be done. The attitude one displays when asked to do this sort of menial task can be very revealing. Some people get irritated because such a task is “beneath their intelligence”. These people are usually poor team players who you definitely do not want to hire.

The most important aspect of team work is communication. Yes, even more important than coding. Reversing a btree is a simple problem but the real work is in explaining what is being done and why. It is weird that even though communication is so important, very little effort is spent in teaching it (in many schools it is not taught at all). Being able to explain how to implement this kind of simple task is important, because a large fraction of your work consists of describing your solutions and ideas to other people. It does not matter if you can come up with the greatest ideas in the world if you can not make other “get” them.

Writing the code

Writing the code to reverse a btree should not take more than a few minutes. It is not hard, anyone who has done basic CS courses can get it done in a jiffy. But writing the code is an almost incidental part of the interview, in fact it is just a springboard to the actual problem.

Things tested after the code is written

Once the basic solution is done, you will get asked to expand it. For a btree questions might include:

  • If you used stack based iteration, what if the btree is deeper than there is stack space?
  • What if you need to be able to abort the reversal and restore the original tree given an external signal during reversal?
  • What if the btree is so large it won't fit in RAM? Or on one machine?
  • What if you need to reverse the tree while other threads are reading it?
  • What if you need to reverse the three while other threads are mutating it?
  • And so on.

As we see even the most seemingly simple problem can be made arbitrarily hard. The point of the original question is not to test if you can reverse the tree, but find how far you can go from there and where is the limit of what you don't know. If you come out of an interview where you are asked to reverse a tree and the only thing you do is reverse the tree, you have most likely failed.

When the edge of knowledge has been reached comes the next evaluation point: what do you do when you don't know what to do. This is important because a large fraction of programming is solving problems you have not solved before. It can also be very frustrating because it goes against the natural problem solving instinct most people have. Saying “honestly at this point I would look this up on Google” gives you a reply of “Suppose you do that and find nothing, what do you do?”.
Different people react in different ways in this situation. These include:

  • freezing up completely
  • whining
  • trying random incoherent things
  • trying focused changes
  • trying a solution that won't solve the problem fully but is better than the current solution (and explicitly stating this before starting work)

This test is meant to reveal how the person works under pressure. This is not a hard requirement for many jobs but even then those who can keep a cool head under stressing circumstances have a distinct advantage to those who can't.

Points of criticism

There are really only two reliable ways of assessing a candidate. The first one is word of mouth from someone who has worked with the person before. The second one is taking the candidate in and make them work with the team for a while. Neither of these approaches is scalable.

The coding interview aims to be a simplification of this process, where the person is tested on things close to “real world” problems. But it is not the real world, only a model. And like all models, it is a simplification that leaves some things out and emphasizes other things more than they should.
The above mentioned lack of Internet connectivity is one. It can be very frustrating to be denied the most obvious and simplest approach (which is usually the correct approach) for a problem for no seemingly good reason. There actually is a reason: to test behavior when information is not available, but it still may seem arbitrary and cold, especially since looking up said information usually yields useful data even if the exact solution is not found.

A different problem has to do with thinking. The interviewers encourage the applicant to think out loud as much as possible. The point of this is to see the thought process, what approaches they are going through and what things they are discarding outright. The reasoning behind this is sound but on the other hand it actively makes thinking about the problem harder for some people.

Every time you start explaining your thought process you have to do a mental context switch and then another one to go back to thinking. Brain research has shown us that different people think in very different ways. Talking while pondering a problem is natural for some people. It is very taxing for others. This gives a natural advantage to people who talk while thinking but there is not, as far as I know, any research results that these sort of people perform better at a given job.

Mental context switches are not the most problematic thing. For some people explaining their own thought process can be impossible. I don't know how common this is (or it it's even a thing in general) but at least for me personally, I can't really explain how I solve problems. I think about them but I can't really conceptualize how the solution is reached. Once a solution presents itself, explaining it is simple, but all details about the path that lead there remain in the dark.

People who think in this manner have a bit of an unfortunate handicap, because it is hard to tell the difference between a person thinking without speaking and a person who has just frozen with no idea what to do. Presumably interviewers are taught to notice this and help the interviewee but since all books and guidelines emphasize explaining your thought process, this might not always succeed perfectly.

But does it really matter?

That's the problem with human issues. Without massive research in the issue we don't know.


  1. "This is not a hard requirement for many jobs but even then those who can keep a cool head under stressing circumstances have a distinct advantage to those who can't."

    This is assuming everything else being equal. You cover this too later in your post, but really the main problem with tests - any sort of tests or metric really, is that they tend to make people focus on the wrong issues, i.e. they focus on what is being tested instead of what should be tested.

    Also I'd object to word of mouth as a reliable method. What you get there is hearsay that you need a model of the middle-man to evaluate. :)

    One way around the: THINK ON COMMAND AND GIVE US THE ANSWER NOW! pressure is to let people work on a problem at home, e.g. over a weekend, and then interview them.

  2. Yep. All of this goes to show that interviewing effectively is really hard.