Early August. Found job posting through HackerNews, emailed them my resume and cover letter. They quickly got back to me. First round was 30-45 minutes of skyping, two programming questions. The first had to do with graph traversal, the second was calculating the sum of Manhattan distances between a certain number of points. I was then asked to optimize it. The second round, a 2-hour coding challenge, was to write a calculator. Mine passed their test cases, but either wasn't good enough or I used poor coding practices because I was rejected.

B Tree related questions, plus probability questions, and a lot of questions related with algorithm& data structure. and also some questions with code optimization. it might be a difficult Challenge if you never touch code optimization with computer architecture.