Amazon Software Development Engineer interview questions

Got an initial interview from the company due to a friend's referral. The initial interview consisted of a phone screening. I also got an on-campus interview afterward, which was unrelated to the phone interview. The on-campus interview consisted of a first-round interview on a Monday. They then called on Monday night to inform me that I had made the second round, which would take place in the form of three interviews on campus over the next two days. All the interviews were very technical, with one or two behavioral/resume questions thrown in. The interviewers themselves were pretty laid-back and fairly helpful, they weren't trying to confuse me or make life difficult. They told me that they would get back to me by the end of the week, but took until the middle of the next week to get back to me. I wasn't offered the position, even though I thought I did reasonably well on the interviews.

I submitted my resume to an Amazon recruiter during a career fair and I was contacted for an interview a couple days later. The HR person who's in touch with me always takes about a week or two before responding to any of my emails, so scheduling the interview itself was a pain. She asked me when I'd be available for the next 2 weeks, I reply right away, she replied two weeks later apologizing for missing my time and ask me for the next 2 weeks I'm available. This went on for about a month. Finally I was passed to another recruiter, who responded a lot faster and scheduled my interview within a week. The interview itself was technical, a couple programming problems and some OOP design concepts. I was asked to code on paper while explaining my thought process on the phone, which I always find hard to do, but I thought I did okay. I didn't have any problems with the coding itself. I was contacted for a second round of interview a week later, and scheduled the interview 2 weeks after the call. It was another technical phone interview with Java concepts and programming problems. Unfortunately, the interviewer had a thick accent and the phone connection wasn't very clear (I heard a lot of background noises from his end) so I found it hard to understand him. He asked me about how to design the system at Amazon for displaying the availability of an item; this one I couldn't answer (more detail about the question below). I was also asked if I knew scripting languages/PERL. I was asked a little bit about my school projects, if there are any challenges that I faced, and asked about what I think is the hardest course.