This is truly an exercise in clarifying requirements and a bit of computer science trivia. Great for competitive coders, questionable for interview prep. Not impossible that someone will interview with this problem but very rarely would this be an appropriate measure of problem solving and job fit.

Go ahead and try it to familiarize with the problem but if you get stuck, just start reading instead. You'll learn more than by thinking in circles.

I also really appreciate the amount of thoughtful responses here instead of mindlessly posting solutions!!! Way to go HackerRank community!!!