Well, that was a month. Congrats everyone who has reached the end, and thanks to everyone who has contributed solutions and advice.
Sometime in January I will create a megathread for visualizations. If anyone has any other ideas, happy to hear them, otherwise, take a well earned 11 month rest until next year :D
I liked 19 as well. Adding memoization and seeing the runtime drop to near zero was so satisfying.
My main learning takeaway was that I am not fully aware of all the various inbuilt iterators in rust, so exploring those was quite valuable.