Had a technical phone screen. I provided the solution with O(1) runtime and O(n) space. The interviewer was fixated on optimizing the solution to achieve O(1) space. Looking at the solution online, I doubt if someone interviewing for a QE role will be able to answer it without already knowing the solution. In my professional experience of over 8 years, I have never come across such complex problems at work as a QE. If Confluent QEs are solving such problems in their day to day, Kudos!