[davelnewton]

I'm having a hard time reconciling the first two sentences.

If it's intuitive then where was the pressure from? And if you had 20 minutes there wasn't much of a time constraint.

The last sentence is counter to your original post, which was that because of this part of the interview you didn't get the job (which is possible, but I'm still skeptical).

I'd spend some time practicing "mini presentations" where you take a problem like this, and pretending like you're in that situation. Code on a whiteboard (or some paper on the wall). Think out loud so the interviewer knows where you're at. Practice how to write code for someone else to read, and so you have an idea of what patterns or anti-patterns you exhibit.