Phone interview included the introduction of the past work, how do you know AppNexus and why you are interested in this position ( This question is repeated in every step of the interview by everyone - you'd better prepare a good strory ), and how to test a specific object ( you need to consider all of possible input scenery ). Onsite included one non-tech warm-up, three tech interviews and one non-tech with Director and the last with HR. The first two tech interviews were similar, they introduced the business model they used and asked how to test it. The third tech interview was also about algorithm and how to test it, code review of a solution. In sum, all of the testing question need you to answer " what's the possible input? " P.S. the next room also had an interview at the same time, it was about Software Engineering. The code on the board was about consumer and producer, a classic problem in multithreading. Comparing with this question, I can't complain my interview was hard.