Unfortunately, it was the worst interview of my life. It is not about the question itself but an unprofessional approach to feedback + evaluation. First of all, in every stage of communication, it was clearly mentioned that I would get feedback in 48 hours either way. But it was not the truth. It took 5 days to get the response and only after I dropped an email about asking the status. And the feedback itself was totally baseless. The task was to create a "urlShortener". My solution was fully capable and working in O(1) time complexity. I even did the TDD approach and my tests are also there and running. I was rejected due to using "non-thread-safe data types, mutable statements and objects" - There was no requirement for multithreading in task definition, and in the interview preparing document it was mentioned to avoid unnecessary overhead such as multi-threading and interfaces. Another feedback was due to some tests being ambiguous - which again I strongly disagree. The task was simple, so the tests were simple and clear.