I think leetcode tends to be a randomized signal because if you've seen the problem before then it's easy, but if it's an unfamiliar problem (or class of problem) then you're basically being asked to be Knuth/Dijkstra/et al., but faster. Algorithm puzzles are fun, but they bear little resemblance to actual work.

I think doing very basic, simple things is a better test. I also like asking candidates to explain something that they already understand well. And for code, I like reading it and also getting an explanation about why things were done the way they were.