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diff --git a/leetcode/1-two-sum/solution.rkt b/leetcode/1-two-sum/solution.rkt
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+#lang racket
+
+;; https://leetcode.com/problems/two-sum/description/
+
+(define/contract (two-sum nums target)
+  (-> (listof exact-integer?) exact-integer? (listof exact-integer?))
+  (define h (make-hash))
+  (define (loop nums index)
+    (define n (car nums))
+    (define diff (- target n))
+    (if (hash-has-key? h diff)
+        (list index (hash-ref h diff))
+        (let ()
+            (hash-set! h n index)
+            (loop (cdr nums) (+ 1 index)))))
+  (loop nums 0))