(defn gen-brackets [n]
  (->> (concat (repeat n \[) (repeat n \]))
       shuffle
       (apply str ,)))

(defn balanced? [s]
  (loop [[first & coll] (seq s)
	 stack '()]
    (if first
      (if (= first \[)
	(recur coll (conj stack \[))
	(when (= (peek stack) \[)
	  (recur coll (pop stack))))
      (zero? (count stack)))))

user> (->> (range 10)
     (map gen-brackets ,)
     (map (juxt identity balanced?) ,) vec)
[["" true]
 ["[]" true]
 ["[[]]" true]
 ["[][[]]" true]
 ["[]][][][" nil]
 ["[[[[[]]]]]" true]
 ["]][[][][[[]]" nil]
 ["[]]]][[[[]][][" nil]
 ["][][[]]][[][][][" nil]
 ["][][]]][]][[[][[[]" nil]