HR call for 45 min, coding challenge over the weekend.

Solved with all of my knowledge except O(1) constraint.

Remember to solve the challenge with all the stuff like concurrency test, performance tests and clean code, then may be you will get chance to go for next rounds.

And also if you are not ready to do above things - just don't waste your weekend. No matter how elegant, how beautiful, how reactive, functional you develop the rest API. They ain't gonna care

The application is a statistics-based REST service.