Mastodawn

Memoization actually worked for this one. My first instinct was to just crawl the graph with a memo, and it fortunately did the right thing. It was effectively an exhaustive depth-first search. Really not much to discuss.

My code is not nice or clean, but after yesterday's, I can't even be assed to clean it up or properly comment it. I barely gathered the gumption to pass errors up, and when I started writing it, I was just throwing unwrap() everywhere.