Amazon Sr. Software Engineer interview questions

I interviewed with the Devices and MAKO groups. One thing I have to say, Amazon has been working hard to improve the interview experience and it shows. The interview process is greatly improved over what it was several years back when I went through the process. I was on the "accelerated interview process" because of the amount of experience that I have. So, I first had to complete an online assessment, then an online phone interview, then an onsite interview with 4 engineers. The Amazon recruiter told me that if I "mess up" with one of the onsite interviewers to not worry, just "shake it off" and go on to the next one. Because only 2 or 3 needed to be good interviews to get hired. I don't think this is true. I believe it was the one that didn't go well that didn't get me the offer. So, the online assessment went great for me. The phone screen was successful. 3 of the 4 onsite interviews went very well and I felt confident in them. The one that didn't go so well was the second interview. For one, the interviewer had a very strong Indian accent and I had to ask him to repeat himself frequently. The question he asked should have been simple and straight forward, but what threw me off was his behavior. From early on in the interview, when I would turn around and look at him, he would be dramatically shaking his head as if he was completely disapproving of me from the get go. I can't tell you if he was pre-judging me over something, if he has a nervous tic, or if the head shaking was supposed to be part of the interview (testing me under pressure, so to speak). So, I'll include the question that I screwed up below. I know I did well on the other interviews because when they called me to give me the news that they were not giving me an offer, they stressed to me TWICE that there were "many very positive" things about my interview, so they "strongly encourage me to interview again in the future". But they couldn't disclose why they were not giving me an offer. Gee, thanks. Well, I'm not getting any younger and I'm off to interview with some other large employers in the area.