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#lang racket
;; https://leetcode.com/problems/valid-parentheses/
(define (is-left-paren c)
(or (eq? c #\u28)
(eq? c #\[)
(eq? c #\{)))
(define (right-paren c)
(cond ((eq? c #\u28) #\u29)
((eq? c #\[) #\])
((eq? c #\{) #\})))
(define/contract (is-valid s)
(-> string? boolean?)
(define (loop s stack)
(if (= 0 (string-length s))
(null? stack)
(let ()
(define c (string-ref s 0))
(define remain (substring s 1))
(if (is-left-paren c)
(loop remain (cons c stack))
(if (null? stack)
#f
(let ()
(define stack-top (car stack))
(if (eq? c (right-paren stack-top))
(loop remain (cdr stack))
#f)))))))
(loop s '()))
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