Although I am rating the process as difficult, I think the questions were reasonable and similar to what you'd find in LeetCode medium level -- a few straightforward, a few more challenging but doable with enough practice/hints from interviewer. Focus on trees, data structures, etc. One of the questions was coding based but design in nature, i.e. building a class. I did not get a strictly design question. Most of the engineers I interviewed with previously worked at Microsoft and Amazon, so expect questions that those companies would normally give.