Memoized version of 4clojure problem 101

joly-4clojure-solution101.clj

(let [mem (atom {})]
  (fn lev [s t]
    (cond
    (empty? s) (count t)
    (empty? t) (count s)
    :else (if-let [e (find @mem [s t])]
            (val e)
            (let [ns (rest s)
                  nt (rest t)
                  ret (if (= (first s) (first t))
                        (lev ns nt)
                        (min (inc (lev s nt))
                             (inc (lev ns t))
                             (inc (lev ns nt))))]
              (swap! mem assoc [s t] ret)
              ret)))))