It’s an algorithm question. If somebody gave me your solution in an interview I would ask them to solve the problem without a dependency. These questions are about demonstrating your ability to code whilst understanding space & time complexity.

Can you list ones you found valuable?

Postfix wasn’t in my university degree, nor do I think it should be. It’s useful to know about SMTP but it’s like saying you need to know the history of brick manufacturing to be a material engineer.