I don't think you need to worry about this. A simple search for "binary search" shows 20 different questions. If you can't do the 1st bst problem and you look at the solution then that's okay! You have 19 more to try it on. Even if you do all 20 then come back in a month and try them again. There's no way you're going to be able to memorize that many variants. And that's just 2% of their entire set of questions.
To really answer your question, I prefer to just hit the "random problem" button and see if I can solve it in 15 minutes with an optimal solution. If I struggle, then I just go read the solution (to understand how I should have solved it) and try a couple related problems to make sure I'm not just memorizing it and I actually understand the methodology behind solving it.
Good luck!