Applied online through their website and got an email asking me to take a HackerRank challenge about a week later with a semi-personalized email from a recruiter.

The HackerRank wasn't that tricky: implement a very basic key-value database with rollback support.

They gave two weeks to do the HackerRank, but stated the expectation was that I would finish it within 2-3 hours (which I did).

A few days after I submitted, they denied me over email.

As far as I'm aware, I met all of the stated runtime complexity requirements. I passed all of the provided test cases, and I also did some manual testing to ensure correctness. So, I'm confused as to why I got rejected.

Create a key-value database that supports transactions and rollbacks.