Amazon Software Development Engineer interview questions

I applied for some job post on amazon.com career site. And i was contacted to go for my first interview in my university's career center. On that evening, I was told that i will be proceeding to the 2nd round, which is another 3 interviews two days later. All 4 of the interviews are technical based and 45 minutes each. I got an email that they would like to make me an offer the day after the 2nd round.

One interview by phone. I thought I've answered all of the questions correctly but did not pass to next phase. Asked about matrix, memory optimization, complexity... He asked a lot of questions and in the end, I had to write an algorithm. He hanged up the call and gave me 10 minutes to solve the problem. I could do it in less time and he called me and asked to explain my algorithm. Since it was an international call, I was not listening the interviewer very well too, but he was incredibly calm and repeated everything patiently.