I was given a code challenge, which is quite difficult since you needed to find an O(n) solution. After that I had an hour long phone interview, which I thought went well. It was mostly data structure (hash map), multi-threading, behavioral and C++. Practice hard questions in algorithm section in hacker rank for the challenge section. Read up hash table, collision handling mechanism. Multi threading basics and how to implement lock free hash-maps. Also do your research about the company and feel free to ask questions.