Task:

 Generate a string with   N   opening brackets   [   and with   N   closing brackets   ],   in some arbitrary order. 
 Determine whether the generated string is balanced; that is, whether it consists entirely of pairs of opening/closing brackets (in that order), none of which mis-nest.


Examples

   (empty)      OK
   []           OK   
   [][]         OK   
   [[][]]       OK 
   ][         NOT OK
   ][][       NOT OK
   []][[]     NOT OK



 
#lang racket
 
(define (generate n)
  (list->string (shuffle (append* (make-list n '(#\[ #\]))))))
 
(define (balanced? str)
  (let loop ([l (string->list str)] [n 0])
    (or (null? l)
        (if (eq? #\[ (car l))
          (loop (cdr l) (add1 n))
          (and (> n 0) (loop (cdr l) (sub1 n)))))))
 
(define (try n)
  (define s (generate n))
  (printf "~a => ~a\n" s (if (balanced? s) "OK" "NOT OK")))
 
(for ([n 10]) (try n))
 