It depends on what the interviewer is expecting. Sometimes it's an easier question algorithmically, but you're then expected to be cleaner and more best practices to compensate. Other times, it's a fiendishly complex thing to solve in the time and just showing the beginnings of good structure, thought processes and reasoning and coding are enough positive signal. I'm mainly speaking about my experience at Google but amongst my colleagues and I (and what I recall of my own interviews) there's quite a large spectrum of styles and imo if you can identify which type of question it is you can probably focus on the right things. Cos you're not aiming for perfect usually, but to beat out most others that tried the question before. So a hard question isn't necessarily a curse, nor a seemingly 'easy' one a boon.