3n + 1 problem

threen.clj

;; 3n + 1 problem
;; https://en.wikipedia.org/wiki/Collatz_conjecture

(defn threen
  ([n]
    (threen n 0))
  ([n k]
    (let  [newK (if (>= n 1) (+ k 1) k)]
      (if (= n 1)
        newK
        (if (even? n)
        (threen (/ n 2) newK)
        (threen (+ (* n 3) 1) newK))))))

(defn max-threen [i j]
  (print i " " j " "
    (apply max (map #(threen %) (range i j)))))

;;

(max-threen 1 10)     ; 1 10 20
(max-threen 100 200   ; 100 200 125
(max-threen 201 210)  ; 201 210 89
(max-threen 900 1000) ; 900 1000 174