A-M baby

Task/Longest-common-subsequence/Racket/longest-common-subsequence.rkt

@@ -0,0 +1,21 @@
+#lang racket
+(define (longest xs ys)
+  (if (> (length xs) (length ys))
+      xs ys))
+
+(define memo (make-hash))
+(define (lookup xs ys)
+  (hash-ref memo (cons xs ys) #f))
+(define (store xs ys r)
+  (hash-set! memo (cons xs ys) r)
+  r)
+
+(define (lcs sx sy)
+  (or (lookup sx sy)
+      (store sx sy
+             (match* (sx sy)
+               [((cons x xs) (cons y ys))
+                (if (equal? x y)
+                    (cons x (lcs xs ys))
+                    (longest (lcs sx ys) (lcs xs sy)))]
+               [(_ _) '()]))))