Revolut Software Developer interview questions

I didn't apply, a recruiter reached out to me through LinkedIn. First interview was with HR asking about my experience etc. Second interview was to implement a thread safe shortener url following SOLID and everything has to be tested. Even though I implemented it in a thread-safe way, following SOLID principles, the feedback I got was that I had difficulties understanding the tasks and only detailing the negative points of the interview, which seems a harsh for how the interview went. I should have expected this from the other reviews i have seen here. Also, the technical interviewer had a strong russian accent that was not easy to understand.