I was contacted by a Google recruiter and went directly to an on-site interview, I believe due to a strong internal referral. I spent a week preparing for the interview. On interview day, there were 5 rounds of technical interviews plus lunch. In total, there were 9 questions. I answered all of them correctly, though for one, I realized it was a dynamic programming problem but didn't have time to figure out the details because I was only given 5 minutes to work on it (the next interviewer was waiting). I did use hints for solving some of the problems. I felt really good about the interview. A week later, I was rejected, and the recruiter said she couldn't give me any feedback.

I can only think of two possible reasons for the rejection:

1. They only hire engineers who can come up with a perfect solution in 5 minutes, which is not what they claimed in their interview tip video, where they said they want to see the brute-force solution first and observe analytical skills.
2. They rejected me for factors outside of the interview.

I felt the second reason is more likely in my case, and they rejected me due to my experience, age, and/or lack of an Ivy League education. In the past, I've received offers from top companies with worse interview performances. This is ridiculous; they seem to have a mysterious formula to determine whether you are a good fit. They can pretty much figure it out from my resume, so why waste my time and energy on the interview! It cost me $500 to take PTO.

A variation of the string matching problem.