People will just game this as well. If you already know the answer to a question coming in because you leetcode all day you could easily ask a lot of clarifying questions, identify edge cases, and give brunt force/alternative approaches before solving the question. A smart person will use that opportunity to highlight their communication skills and get an even higher grade on the interview
Our questions aren't on leetcode and we aggressively monitor for leaks (including on Blind). You're also missing the point: we're intentionally avoiding the questions which boil down to a binary question of "do you know this algorithm or not?". Most of our questions are open-form, asking the candidate to implement a widget or interface with an API. They usually don't involve big-O or any data structure tricks. Unless you've seen the exact problem before leetcode won't help aside from general programming practice.
Also I will say that the few candidates who've tried lying about seeing a question before are not as good at acting as they think they are. Not only do you have to pretend that you've never seen the question before but if you don't stumble, second-guess yourself, refactor parts of it, etc. it's usually pretty obvious.
Now, if someone's a great actor and they've seen all four problems before and manage to impress the hiring manager -- well, what can you do. But I don't think there are very many of those.
