Group and Count
Hey! Here's my solution to the exercise.
My approach was to solve the problem recursively, breaking the problem up into smaller, similar subproblems. This was a natural-seeming approach once I realized that I was effectively solving the same problem on multiple levels of a tree structure. I defined the base case as when the array of keys was empty. In this case I simply returned the size of the array. Otherwise, I called Enumerable#group_by on the array, grouping by the value of the first key. Then, I mapped my group_and_count function (a recursive call) over the new hash.
There's still one problem with that, though, which is that Enumerable#map always returns an array, even when called on a hash. Not what we want! So I defined a Hash#hmap method that returns a hash with the block mapped over the values of the receiver while leaving the keys intact. I did this by calling #map on the hash, with a block that returns a pair The one-liner with [k, prc.call(v)] (prc being the block passed into hmap... yo dawg I heard you like blocks).map was cute but I rewrote it with an each block for better space/time efficiency. As much as I like functional programming...
As far as I can tell the code works as intended. I monkey-patched Array and Hash directly for convenience's sake but that's probably something I'd avoid in production code. It surprises me that nothing like #hmap seems to be built into Ruby.
Usage
Git clone, bundle install, bundle exec rspec.