I would also be interested in some more detail, especially: "On the phone interview, I received a pretty challenging problem—I remember thinking when I got it that I was going to mess it up. But somehow I solved it with ease, and all of my outputs were correct."

It's kind of hard to get a handle on what this means; on one hand the poster is quite obviously remarkably intelligent (ref: bio etc), on the other hand he has very little CS experience. The reader is left to imagine a problem that is just within their own reach to solve, while it might have been a very basic algorithm question...

There's InterviewBit[1] that helps you prepare. You don't have to sign-up to read problem descriptions.

[1] https://www.interviewbit.com/all-problem-list/