(defn longest [xs ys] (if (> (count xs) (count ys)) xs ys))

(def lcs
  (memoize
   (fn [[x & xs] [y & ys]]
     (cond
      (or (= x nil) (= y nil)) nil
      (= x y) (cons x (lcs xs ys))
      :else (longest (lcs (cons x xs) ys)
                     (lcs xs (cons y ys)))))))