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



(define (balanced-brackets string)
  (define (b chars sum)
    (cond ((< sum 0)
           #f)
          ((and (null? chars) (= 0 sum))
           #t)
          ((null? chars)
           #f)
          ((char=? #\[ (car chars))
           (b (cdr chars) (+ sum 1)))
          ((char=? #\] (car chars))
           (b (cdr chars) (- sum 1)))
          (else
           (#f)))
  (b (string->list string) 0))
 
(balanced-brackets "")
 
(balanced-brackets "[]")
(balanced-brackets "[][]")
(balanced-brackets "[[][]]")
 
(balanced-brackets "][")
(balanced-brackets "][][")
(balanced-brackets "[]][[]")
 