What are the hardest leetcode problems?

I worked with someone today who spent the vast majority of the past week, only studying graph problems for one of the hardest companies to get into and they weren't asked any graph problems and we're only asked string problems. You are missing the point in that you still need to learn graphs, dynamic programming strategies and much more complex approaches to solving problems. But people have attendancy of sitting in their room cranking through these problems alone and thinking they know how to do them when in fact they've just really memorized or convince themselves they understand them without really getting signal from the outside that they do until the interviews. An engineer needs to learn how to solve problems in general. You are paid the big bucks to solve new problems not to regurgitate hard solutions from Leetcode. I'm my opinion you are going to have a stressful and hard time otherwise and have a lower probability of passing I helped grow Facebook from 200 engineers to 10,000 engineers, did hundreds of interviewers, helped create an interview type, helped train interviewers, so I'm just giving my advice from my perspective, you can always do you and do what you want to prepare and whatever works for you!