I had a phone interview with a senior member of the team I was interviewing for. It started with the job description and a technical/design problem I had to solve. Then, there were some bitwise operations questions. I am sure I answered those correctly, as the interviewer seemed convinced by my explanations. Overall, the interview felt like it went really well, judging by the interviewer's responses. However, three days later, the recruiter said they would not be proceeding. No feedback was shared even after I asked.

Find the number of set bits.

Improve the complexity to the number of set bits.

Are there any other approaches you could think of? Which one would you use and when?