OA Round

Question # 1: Given positions of N items, find closest M items to the origin
Question # 2: Maximum number of partitions of a string such that no two partitions have same character.

Experience: Both of these questions can be found in top 100 most frequently asked Amazon questions on leetcode. I could solve both them within an hour. Thanks to leetcode!
Learning: OA looks easy to pass if you have solved top most frequently asked questions on LC.

HR round/Screening

Asked details about experience and 2-3 behavioral questions.

1st Virtual Onsite Round

Question: Print all the levels of a tree in reverse order
Experience: If you know the BFS, this is an easy question, but interviewer simply rejected my first approach and wanted me to try a different one (Even though my first approach was efficient and simple to implement). I guess that he wanted to check if I could come up with a new approach. Though I could implement it, I was coding till last moment.
Learning: Better to know at least 2-3 approaches of each problem you practice.

2nd Virtual Onsite Round

Question: This one is bit difficult to describe fully here. But to give you an idea, I was wasked to design and implement a class to simulate a game model in which multiple objects are going at different speeds from bottom to upward. The objects do not collide, they wait if some other object is in front of them. In short, the class should be able to maintain the state of the game.
Observation: Most of the time wasted in just trying to understand the problem. Though I could implement it, but again I was coding till last moment. Also, this round was the most difficult to predict if the interviewer was satisfied or not.
Learning: Though it is important to understand the question well, we should keep an eye on time too. I observed that interviewers do not care much if you waste lots of the time in just discussion, but at the end you will suffer because you will not have sufficient time to code. Ideally you should try to save around 30 min to code.

3rd Virtual Onsite Round

Question In a very long genome sequence, find a given list of patterns. Return True/False depending on if you could find all of the patterns or not. The catch is that to access the sequence, you get just one API, getNextItem(). You cannot store the geome fully, as it is really huge.
Experience: Again to me it took a while to understand the problem. The problem was doable for me if asked normally without any pressure of time. But it was very difficult for me to solve it perfectly in 20-30 minutes. I guess, this one was what they call "bar raiser" round. I could come up with an approach and code it. But there were some bugs/missing cases in my code.
Learning: Amazon wants excellent coders. The recruiter may say you many times, that they ask just medium type problems, but the truth is that in atleast one of their rounds, they are going to ask a hard type problem. Please practice A LOT if you want to go to Amazon :)

4th Virtual Onsite Round

Question: Design a reservation system of all the restaurants in US. The design should be scalable as in future we may have to add more countries in the system.
Experience: I felt that I did very good in this round. Thanks to my previous practice of system design.

Overall Learning

1. Practice, practice and practice. There is no shortcut. You may be a star performer in your current job, but unfortunately for companies like Aamazon it does not matter unless you are an excellent coder who can solve medium to hard questions within 20-30 minutes, + a decent communicator + a decent system designer.
2. Do not simply ignore hard problems assuming that they may not be asked because of time limit in the interview. I did this mistake (actually because of the limited time I had for my preparation).
3. They expect you to think aloud while coding or while coming up withan approach. This is a challenging thing to do, so practice that too :)
4. Amazon asks lots of behavioral questions and it takes time to answer if you have not prepared enough. Interestingly, they can even reject your first answer and ask you to share another experience or story. This may waste lots of time. But they do not care much about it. It is our responsibility to prepare well and save time in the interview.

In short, you need a focused preparation of 6 months to one year. If some people cracked it in short time, the reason could be that they already had a great foundation (by education or self study) or they are really smart people :)