Meta Software Developer interview questions

Was contacted by recruiter in LinkedIn and started interview process. He was transparent, prompt and supporting during the process. He updated me every on every step and details and he was prompt: getting feedback after 3 hours from phone interview speaks for itself. Then I had on-site interview, three interviewer liked my answers, system design really like my design and thinking, but apparently one of the interviewer didn't like my coding style and I was asked to come for one more interview just to check the coding part again. So tip: use whiteboard better, apparently working and clean code is not enough it needs to look clean on board as well :) Overall process was positive. But, I had a feeling that some interviewers are really un-experienced and focus on wrong things. Also big difference from other companies: they expect you to be really fast (read - you need to know answer upfront). If you don't know answer upfront, but come up with solution after thinking, trying, making misatkes and optimizing - it doesn't count, at least it didn't in my case. Try to solve my problem below (if you don't know answer already), implement, test it on board in 15-20 minutes. Advice for FB: train your interviewers to focus on skills, NOT memorized skills. I have offer from big companies/competitors, so I will just go with them.