What are the hardest leetcode problems?

If you know the question number off the top of your head of a question that you think is the hardest, you are preparing for interviews wrong and cramming Leetcode way too hard. Unless you want to be in programming competitions and aren't trying to get hired. If you learn the right techniques for how to approach problems, and have practiced the fundamental techniques, no problem should be hard. The only exception is problems that solely rely on an obscure computer science algorithm that isn't used anywhere else except for this problem, but you shouldn't be studying those on Leetcode, except for fun.