ICPC2011 ProblemB

ちょっと時間がとれないので簡単な物ですみません。

(defun balancingp (str)
  (let ((ch-list (coerce str 'list)))
    (labels
	((%balancing-p (ch-list stack)
	   (if (char= #\. (car ch-list))
	       (null stack)
	       (cond
		 ((or (char= #\[ (car ch-list)) (char= #\( (car ch-list)))
		  (%balancing-p (cdr ch-list) (cons (car ch-list) stack)))
		 ((char= #\) (car ch-list))
		  (if (equal #\( (car stack))
		      (%balancing-p (cdr ch-list) (cdr stack))
		      nil))
		 ((char= #\] (car ch-list))
		  (if (equal #\[ (car stack))
		      (%balancing-p (cdr ch-list) (cdr stack))
		      nil))
		 (t (%balancing-p (cdr ch-list) stack))))))
      (%balancing-p ch-list ()))))

(defun main ()
  (loop
     for line = (read-line)
     until (string= "." line)
     do
       (format t "~A~%"
	       (if (balancingp line) "yes" "no"))))

スタックにプッシュしていって、順番にポップしていってという感じです