Y箱

また出てきた素数。
素因数分解して解の員数を出して解いているんだけど、素数列をその度に作っているので非効率。
今後のためにも、素数列は取っておけるようにしようかなぁ。

;;
;; Problem 5  : 2011/4/7
;; prime-nums-under : defined above

(defn factors [n]
  ;; (foctors 12) => (2 2 3)
  (loop [factor-list ()
     target n
     prime-list (prime-nums-under target)]
    (if (or (= target 1) (empty? prime-list))
      (reverse factor-list)
      (let [one-prime (first prime-list)]
    (if (zero? (rem target one-prime))
      (recur (cons one-prime factor-list) (/ target one-prime) prime-list)
      (recur factor-list target (rest prime-list)))))))


(defn fusion
  ;; (fusion '(2 3 3) '(3 4 4)) => (2 3 3 4 4)
  ([list1 list2]
     (fusion list1 list2 ()))
  ([list1 list2 res-list]
    (cond 
      (empty? list1) (sort (concat res-list list2))
      (empty? list2) (sort (concat res-list list1))
       true
         (let [top-of-1 (first list1)
               top-of-2 (first list2)]
            (cond 
              (= top-of-1 top-of-2) 
                  (recur (rest list1) (rest list2) (cons top-of-1 res-list))
              (< top-of-1 top-of-2) 
                  (recur (rest list1) list2 (cons top-of-1 res-list))
              (> top-of-1 top-of-2) 
                  (recur list1 (rest list2) (cons top-of-2 res-list)))))))

(reduce * (reduce fusion (map #(factors %) (range 2 21))))

;;