Amazon Software Development Engineer interview questions

First Amazon came to my school's career fair, and I spoke with one of their representatives. They asked me a simple question on how to find the second biggest number on a unsorted list and I had to code that as well. Then about 2 months later, I got an invite to their HQ for an onsite interview, and they paid for everything. There were a bunch of people interviewing with them as well. Each of us get four individual 1:1 interviews. Each interviewer is from a different department at Amazon. All of them are technical, and I had to code on a white board for each one of them. They also asked some small questions along with the coding questions for each one of them. Most of them are pretty easy. They said I should get a response within a week, but I didn't, so I emailed them after 1.5 weeks. Then they rejected me and didn't really tell me a reason. I thought I did pretty well. All the interviewers are nice and the experience was pretty good. Seattle is indeed a nice city. I just wish I had gotten the offer.