recur.clj

(ns aoc2017.foo)

(def node-map
  "A tree structure defined as a map"
  {:a [:b :c]
   :b [:d :e :f]
   :e [:g :h :i]
   :i [:j :k]})

(defn count-nodes
  "Returns the number of nodes located under the given node (inclusive)."
  [node-map node]
  (let [children (get node-map node)]
    (if (empty? children) 1
      (inc
         (reduce +
            (map #(count-nodes node-map %) children))))))

(defn main []
  (println (count-nodes node-map :b)))