Project Euler #24 - Clojure

gistfile1.clj

(def digits (sorted-set 0 1 2 3 4 5 6 7 8 9))

(defn nextLex [substr digits]
  (let [lex (apply str substr digits)]
   (lazy-cat
    [lex]
    (loop [s (vec lex)
	   d (sorted-set)]
       (let [i (- (int (peek s)) 48)
             d (conj d i)]
         (if (== (last d) i)
	   (recur (pop s) d)
	   (let [z (first (drop-while #(<= % i) d))]
	     (nextLex (apply str (conj (pop s) (char (+ z 48)))) (disj d z)))))))))

(first (drop 999999 (nextLex "" digits)))