The best answer to the factorial code interview question

Whenever I interview a developer, I always ask them to write code on a whiteboard. Nothing too complicated РI expect everyone uses an IDE these days and the candidate is also probably feeling a little nervous.

The big interview cliche is asking for a method producing a Fibonacci sequence. Even with telephone screening, this still eliminates more candidates than it should. A well-prepared interview candidate should have practised that one already, which means it’s still a useful test. Whether or not someone can do it, there are lots of interesting follow-up questions.

Some candidates get flustered trying to understand the Fibonacci sequence. I’d expect most people to know this already, but I wouldn’t want to reject a potentially-excellent candidate for lacking a bit of maths knowledge. So sometimes I would simply ask for a function to product factorials.

There are several different ways to do this, with the two main options being whether the solution is recursive or not. A simple solution would look something like this:

public int fact(int n)
{
    return n == 1 ? 1 : n * fact(n - 1);
}

One obvious follow-up question to this is what the limits of the function are – using an int means you’ll have overflow problems quite quickly. Does the developer know the class to use to avoid this?

The question simply asks for a piece of code. I would have given bonus points for any developer who mentioned testing before writing. But the perfect response would have been a candidate saying that the code would be different depending on its intended use.

Someone on github has produced an enterprise version of another interview classic, with Enterprise Fizzbuzz. Obviously that is going too far. But there are considerations for even the simplest piece of professional code:

  • Who else needs to work on this? What documentation/commenting is required?
  • How and where will it be deployed?
  • Is any error checking or exception handling required? In the example above – which I would have accepted as correct – there is no handling for the obvious overflow error.
  • Is something this simple suitable for the intended use? For example, a large number of similar requests might be better handled with some sort of question.

All of which is a complicated answer to a simple question. A dev raising these issues in an interview would still need to produce the code – but the discussion that followed would be very different.

Thomas Mann once claimed that “A writer is someone for whom writing is more difficult than it is for other people.” In the same way, the more I think about development, the harder it seems. A factorial example might seem almost insultingly simple – but it’s possible to have a very complicated conversation about it.

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