I knew it related to XOR and AND but the exact process evaded me. Most programmers do not deal with this in their day to day and have not executed these problems since freshman year college. But they let me ramble for 10s of minutes on the topic trying to remember which order of XOR and shifting was correct when in reality it would take 2 seconds to Google it in the work place. I had the fundamentals to the answer but that was not enough and it detracted from the interview as a whole.