Copying the question text verbatim certainly is copyright infringement (and I would guess unlikely to be fair use, but I'm not a lawyer). If you give the problem in your own words, it won't be, just like your solution isn't.
From the 1976 Copyright Act section 101:
https://www.law.cornell.edu/uscode/text/17/101
> A "derivative work" is a work based upon one or more preexisting works, such as a translation, musical arrangement, dramatization, fictionalization, motion picture version, sound recording, art reproduction, abridgment, condensation, or any other form in which a work may be recast, transformed, or adapted. A work consisting of editorial revisions, annotations, elaborations, or other modifications which, as a whole, represent an original work of authorship, is a "derivative work".
It’s a legal construct, but I was never satisfied with that explanation. The gist was that while we could transform things that we learned or did, we could not re-use the ideas that formed the functioning product in other works elsewhere, even by transformation.