Count the 1’s in the binary representation of a decimal number
As I wrote here before, I do a lot of professional interviews, and I discovered that the best questions have more than one solution to them. While having an ideal solution to a problem, you can still ask the interviewees for other ways for the solution and putting extra constraints that can help guide them in either direction (maybe to check their creativity, or if they knew the question beforehand). One of the questions that fit this requirement is the “count the 1’s in the binary representation of an integer”.