Amazon Software Development Engineer interview questions

First round was online coding +apti. Aptitude was kind of moderately difficult and the coding questions were pretty simple. But you must be aware of using libraries in any language so that you don't have to code all of it. For example, my question required sorting and I was coding in C. I had to switch to Java so that I could use Collections.sort(). Couple of weeks past, I got a mail for telephonic interview which was primarily based on my CV. My skills and projects were asked. Apart from that some basic binary tree coding problems were asked. Next, I was invited for on-site interview where I went for rounds after rounds. Until the third round I could see people walking out. For the fourth and the final round, only 5 out of initial 50 candidates remained.