Amazon Senior Software Engineer interview questions

Post resume online and an Amazon recruiter contacted me. Two phone interviews. Got rejection message from the recruiter two days after the 2nd interview. Asked to write code to reverse a string and search a substr. Asked algorithm to produce power set. Did not do well on the second interview since I forgot the formula to calculate combination and slow to react to interviewer's hint to get the answer. Also did not communicate well enough as I was thinking hard. I rated the difficulty level as average because the algorithms would be easy for people freshly out of college. Asked some generak OO design questions about 1. designing a parking lot and 2. components to handle orders. Interviewers seemed to be professional and smart. Thought I did well enough to make it thru the 2nd round but... I thanked the inverviewer after the interview both times but never contacted the recruiter. Not sure if they liked that. Never showed enthusiasm at all during the interview process as I was sure Amazon is not a right place for me. I accepted the interview just to find out what their culture is. Not worth it. No regret at all.