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At Codility we create assessments for hiring software developers. This implies that we also hire developers to create tests for hiring developers. It is like two-star pointers or programming squared. 

Our interview process consists of a Codility assessment in the front, and then good old interviews for the candidates who prove they have the coding knowledge and fundamentals. I want to share four technical interview tips & questions that we use to assess competences in programming during the face-to-face interviews, using CodeLive.

1. Scalability

If you are planning to do something big, it will need to scale well in this way or another. A basic understanding of scalability is assessed by an automated Codility assessment, however we frequently touch the subject in the face-to-face interview too.

We ask candidates to answer a simple programming problem-solving question which has multiple solutions with different simplicity/scalability trade-offs (e.g. compute prefix sums of a sequence of numbers). We assess whether candidate notices the scalability aspect and whether he/she can deliver the working solution with better scalability.

Time permitting we probe deeper, asking “can it possibly be done better?” This question opens discussion about possible optimizations, sometimes going down to the hardware level, but also about intrinsic limitations of computing machinery, which greatly illustrates whether candidate can apply theory to a concrete problem.

2. Aliasing
It is not a revelation that an amazingly high number of professional developers have a lot of confusion about pointers and references. And I don’t mean three-star programming, I mean even simple stuff: if you do “A=B” in Python or Java, what actually happens and do you end with one or two Bs?

We usually ask to answer a simple programming problem-solving question that involves pointers/references, like manipulations on single-link lists. As much as we rarely deal with direct lists manipulation nowadays, we believe it shows whether candidate can deal with real-life scenarios where multiple levels of indirection occur e.g. in data models. (And as David Wheeler used to say All problems in computer science can be solved by another level of indirection, except of course for the problem of too many indirections.)

3. Recursion

Understanding and handling recursion is a sign of programmer’s fitness.

On some jobs this skill will be crucial, on other jobs maybe not so much, but the fact that you know how to do it shows that you have been doing mental push-ups and sit-downs long enough to be ready to take the triathlon challenge.

The classic moderate question here is to take a simple recursive function and ask the candidate to rewrite it into an iterative one.

4. Communication

Coding is not only a way of communicating your requirements to the machine, but also a way of communicating your intent to fellow team members.

For each programming exercise the candidates solve, both on the automated Codility assessments, online CodeLive interviews, and in face-to-face interviews, we look at the code they write and assess whether it speaks to us clearly. If you look at the code and you immediately know what’s going on, that’s a plus, if you don’t that’s a red flag.

Bonus points: 
We created CodeLive to help hiring managers run their technical interviews effectively, by having a shared space to code alongside the candidate. The interviewer has full control to add test cases, change languages, and lock down the interface. The full session is then recorded and scored for other hiring managers to watch and review later, and to be kept on file. 
Our team will sometimes start CodeLive interviews from a candidate’s submitted Codility assessment so that we can ask specific programming problem solving questions about their approach, and test them deeper on their solutions. Other times, we’ll start with a completely new task. 
We like collaborative coding on CodeLive because it gives us deeper developer insights–and a great environment to run our tech interviews.

Greg is the founder and principal scientist at Codility.

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