Algotec interview questions

first technical interview - remove spaces from string in O(n) time complexity and O(1) space. later I was asked about the redo and undo in paint ,had to give 3 ways of implementing it. the Interviewer was very nice and polite. The 2nd interview I asked about producer and consumer event communication with message queue . The question : you have a producer that produces 3 types of events, we have 3 processes that can handle a specific event at a time - synchronize the work. they looked for a signaling mechanism answer with Redis. Later the interviewer expanded the question , don't really remember the specifics , in my opinion he wanted me to design a lightweight Kafka system. both interviews took 2 hours long each.