Biggest problem was acting like I didn't know and getting it really fast.
Total lottery, they could have asked any number of things that I didn't understand.
At least it was for a job I probably wouldn’t have taken.
Well, yeah. That's a big part of the point of the question. Anybody can Google the answer. Do you understand it well enough to explain it, and if you don't, can you walk through your thought process as you figure it out.
My Google and Facebook interviews went much better in comparison. None of their questions were top ten LeetCode ones.
You failed for the same reason you would have failed if you had never heard the problem before. Knowing the answer has nothing to do with why you were dinged.