I was asked to implement LFU cache that supports get and put, no time constraints given. I had done LRU cache before but LFU is a bit tricky and I struggled a lot to come up with an approach, I did finally come up with an approach that involved using a map to keep frequencies and a min heap to get the LFU item, but the interviewer asked to implement min heap from scratch so I couldn't use the one from standard libraries. It me a lot of time and couldn't finish in time, but was able to clearly express my thought process of time complexity of my solution. I was told by the recruiter that I'll be asked a few LP type questions and then a coding question so I wasn't expecting a hard question on the phone screen, I was asked no LP question, not even 'tell me about yourself' in the beginning, we went staright into the coding part, so I feel like I bombed it but still waiting to hear back.