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@@ -0,0 +1,33 @@
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+#lang racket
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+
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+;; https://leetcode.com/problems/valid-parentheses/
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+
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+(define (is-left-paren c)
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+ (or (eq? c #\u28)
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+ (eq? c #\[)
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+ (eq? c #\{)))
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+
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+(define (right-paren c)
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+ (cond ((eq? c #\u28) #\u29)
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+ ((eq? c #\[) #\])
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+ ((eq? c #\{) #\})))
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+
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+(define/contract (is-valid s)
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+ (-> string? boolean?)
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+ (define (loop s stack)
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+ (if (= 0 (string-length s))
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+ (null? stack)
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+ (let ()
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+ (define c (string-ref s 0))
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+ (define remain (substring s 1))
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+ (if (is-left-paren c)
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+ (loop remain (cons c stack))
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+ (if (null? stack)
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+ #f
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+ (let ()
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+ (define stack-top (car stack))
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+ (if (eq? c (right-paren stack-top))
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+ (loop remain (cdr stack))
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+ #f)))))))
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+ (loop s '()))
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+
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