I used recursion to hike from trailheads and collect the coordinates of 9s
that I could reach. I used #uniq to get a score as "how many 9s I can reach
from this trailhead."
By removing the call to #uniq, the #hike method returned one coordinate for
each path. Counting the paths gave me the rating I was looking for.
I tried to port this solution to JavaScript. For fun. The hard part was
replicating Ruby's Array#uniq. I used simple tuples for coordinates, and a
JavaScript Set didn't de-dupe them because it used object identity to find
duplicates. In the end, I converted coordinates to JSON strings before
de-duping, and that did the trick.