(define (balance str)
    (for/fold (closed 0) ((par str))
     #:break (< closed 0 ) => closed
    (+ closed
    (cond
        ((string=? par "[")  1)
        ((string=? par "]") -1)
        (else 0)))))

(define (task N)
(define str (list->string (append (make-list N "[") (make-list N "]"))))
    (for ((i 10))
    (set! str (list->string (shuffle (string->list str))))
    (writeln  (if (zero? (balance str)) '👍  '❌ ) str)))

(task 4)

❌     "[]]][[]["
❌     "]][][[[]"
❌     "][[[]]]["
👍     "[][[[]]]"
❌     "]][[][]["
❌     "][][[[]]"
👍     "[][][[]]"
❌     "]][[][[]"
❌     "[[]]][[]"
❌     "[[][]]]["