geography.clj

(defn edge? [w1 w2]
  (and (not= w1 w2) (= (last w1) (first w2))))

(defn add-edges [m w words]
  (let [edges (filter (partial edge? w) words)]
    (assoc m w edges)))

(defn build-graph [words]
  (reduce (fn [m w] (add-edges m w words)) {} words))

(defn play [m w]
  (if (empty? (m w))
    false
    (not-every? (partial play (dissoc m w)) (filter m (m w)))))

(defn solve
  ([states] (filter (partial solve (build-graph states)) states))
  ([m w] (every? (partial play (dissoc m w)) (m w))))