LinkedIn Software Developer interview questions

I applied through both internal referral and online. Shortly after we hooked up I attended the phone interview. I was asked a simple coding question and some machine learning and system design questions. The interviewer sounded impressed. They arranged the on-site interview with me after Xmas vacation. It was at their NYC office and it's from 10am to 5pm. About 3 interviews had 2 interviewers attended, the others 1. About 3 interviews in the afternoon are remote from their CA office. Most of the questions are system design related to the products they're working on, which I feel are hard to prepare beforehand and tricky to answer. Coding and algorithm questions are relatively few but some were impressively hard. Note the entire process can be very tiring, because 1) it's from 10am to 5pm, 2) most of them require writing on white board a lot, and 3) their office at Empire State building requires authority to pass the door between interview room and restroom. In fact I didn't go to the restroom between remote interviews because I couldn't find someone to hold the door for me. I waited for several weeks and also tried follow-up mails, but didn't hear back from them at all. About 4 weeks later I just decided to call them directly and finally learned that I'm no longer considered.