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diff --git a/leetcode/20-valid-parentheses/solution.rkt b/leetcode/20-valid-parentheses/solution.rkt
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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 '()))
+