Nothing particularly difficult. However, often they do not ask explicitly such as implement a certain algorithm or what does a certain data structure look like. They would instead put you into a real-problem scenario and you gotta choose what's the best data structure and algorithm. After you got an initial answer, they would try to push you further, such as asking how to optimize efficiency, what if the algorithm assumption has changed, what are the extreme cases where there might be a pitfall etc. Be prepared for such followups.

Why did you design [specific part of project] this way, and what difficulties did you have when implementing it?