This problem is terribly written. It reads like one of those high-school level math problems that takes a very simple concept and makes it seem arcane and obscure through its poor use of language and symbolic abstraction.

I think the authors who write these sorts of "math textbook" problems think they're making it clear via symbolic abstraction, but they are in fact completely obfuscating it. This problem is pretty much "creating a running list of books by category" but somehow it manages to blather for paragraphs about c, M, L, and Clojure.

and the inputs don't even match up the ones given in the problem description! Jeebus, this needs to be rewritten. Shame, because it's actually a fun problem.

This isn't a very performanent solution...n^2