so many machines

Specification Gaming and the Coding Interview

Are LeetCode-style interviews helping companies hire the right candidates or has the specification drifted from the goal?

Specification gaming is the unexpected exploitation of the conditions of a test by an agent to satisfy it without actually completing the intended objective. Processes evolve, side-stepping their creators’ motivations, and instead work to maximize their performance in the environment in which they’ve found themselves operating. The Surprising Creativity of Digital Evolution outlines examples in the wild of algorithms exhibiting surprising behaviors by either coming up with novel solutions or abusing latent bugs in the scenario in which it was being trained to maximize a score from a fitness function. In many of these cases the algorithm compromises the intended goal, but like Skynet in The Terminator films destroying humanity to continue its mission of “safeguarding the world,” they precisely follow the specification.

Poor robot. It just wanted to do well and please its creator and didn’t try to destroy humanity even once.

These unexpected outcomes are the result of emergent behavior from unsupervised optimization by actors in a complex system training to maximize positive feedback whose source had either drifted from or had not been seamlessly aligned with the intended goal. In effect, the Roomba’s operator had created conditions in the training scenario that unintentionally asked it to drive backwards.


Meanwhile at the same companies generating hype about the limitless promise of artificial intelligence, engineers work to help their teams build their ranks through coding interviews. The coding interview is a practical examination of a candidate’s ability to write software by posing a problem to be solved extemporaneously. These interviews are a rite of passage for software developers and an obsessed-over hurdle: the only thing between you and that dream job is a blank whiteboard, a smiling-albeit-standoff-ish bespectacled man in khakis, and a binary tree that, you’re told, needs to be inverted. Something akin to the intern qualification exercise in The Social Network, sans the hooting audience, problematic binge drinking, and Sorkin dialogue.

As of this writing, the #1 bestseller on Amazon.com in the computer science category is Cracking the Coding Interview. Widely referred to by its initials by college students and job seekers of all ages, CTCI promises to teach you everything you need to know to perform well in technical interviews at companies like Google, Facebook, et al, via CliffsNotes on computer science topics and 189 example problems and their solutions. It’s a prominent example of a crop of books and startups that have been created to service a market of software engineering hopefuls eager to cram by practicing questions similar to those they can expect on an interview.

What sort of questions are we talking about? Let’s look at an example. Say you’re given a list of numbers [1, 9, 1, 9, 2] and you’re asked to find the number which is in the list only once. Before thinking about it, you’d likely scan this sequence and find that the only number without a duplicate is 2. Splendid. Now, how do you tell a computer to do the same thing?

You could tell the computer to do it the same way you might have done it mentally. For each number, loop through the other numbers, count the number of occurrences, and return the number that only occurs once. This will work quite handily for lists like the one above. With a list of 5 items, this might result in 25 operations of reading the list. Done, right? Well, if I gave you a list of 50,000 numbers, this approach is no longer viable. You can’t physically go through 50,000 items for every item looking for duplicates, and your computer will have to run 2.5 billion operations in order to produce a result for you. This is a brute force approach to solving the problem.

Better, less obvious solutions to this problem exist. One approach is to take advantage of bit manipulation and the exclusive-or operation. Any number XOR zero will equal the original number, and any number XOR itself will equal zero. Thus, if you start with zero, iterate through each number in the list, and XOR them all against that number, you will be left with the number without a duplicate. For this solution, your computer only evaluates each number in the list once and stores only the running tally in memory. Linear time, constant space. Cut the check.

That’s an easy, contrived example. Leetcode is a website that provides a thousand such questions and an interactive environment to test your prowess against a battery of test cases and the time and space efficiency of other users’ solutions. It touts itself as a platform to help people enhance their skills in order to prepare for technical interviews. The question on Leetcode currently holding the lowest success rate asks a programmer to validate whether a given string (such as 12 or -54.9e33 or bicycle) can be interpreted as a decimal number. Provided solutions in the discussion forum for this question range from using Deterministic Finite Automata to parse the string, to regular expressions gymnastics, to “return !isNaN(parseFloat(s)) lol.”

The Internet gives us a window into a subculture of career hacking and coding interview preparation. In communities like the /r/cscareerquestions subreddit, job seekers talk about engaging in mock whiteboard interviews with each other and studying these kinds of problems for hours a day as “grinding Leetcode.” Leetcode organizes their questions by subject area such as breadth-first search, or linked lists, or dynamic programming, and by company. Premium subscribers are given access to the top N questions purportedly asked by top tier technology companies.

Some argue that these coding interviews are far better than the previous trend of why-is-a-manhole-cover-round-style brainteasers in that at least these exercises are practical with well understood answers. Opponents argue that these exercises are too far removed from the quotidian work of software development to be useful, and perhaps even somewhat discriminatory as not all candidates could possibly devote the required “grinding” time to prepare appropriately.

There’s something symmetric and satisfying about technology companies that pride themselves on relentless optimization interviewing candidates that are themselves optimizing for maximum performance against the interview. These filters are, at best, identifying the right, motivated candidates, or at worst, creating a generation of mercenaries with a mentality that to achieve a high paying job in technology one must learn the test and game the specification.

The top all time post on /r/cscareerquestions is titled “Accidentally destroyed production database on first day of a job, and was told to leave…” It recounts the experience of a new graduate following an environment setup document that included production database credentials which they used to run a series of commands starting with DROP DATABASE. The veneer of the coding interview suggests to candidates that hiring managers are wisened monks who spent a lifetime in study for this moment to test your mettle. The power dynamics of the situation belie the reality: that same hiring manager has no idea what they are doing and will hand you a Word document with production database credentials on your first day. No amount of practicing dynamic programming questions will prepare you for that.

Trends in hiring move slowly and while anxiety and backlash over the coding interview seems to be building, there isn’t much in the way of data to show if these interviews are effective or otherwise. For professional developers some navel gazing and soul searching is warranted: is this tool identifying the right candidates or are we unintentionally asking people to drive backwards?