union find fixing

2011-07-16 13:49:46 -04:00 · 2011-07-16 13:49:46 -04:00 · df1ef5b693
commit df1ef5b693
parent 5ff41c40f3
1 changed files with 5 additions and 1 deletions
--- a/union-find.rkt
+++ b/union-find.rkt
@ -31,7 +31,11 @@
 ;; Returns the node that's associated with this element.
 (: lookup-node (forest Symbol -> node))
 (define (lookup-node a-forest an-elt)
-  (hash-ref (forest-ht a-forest) an-elt))
+  (unless (hash-has-key? (forest-ht a-forest) an-elt)
+    (make-set a-forest an-elt))
+  (hash-ref (forest-ht a-forest)
+            an-elt))
+


 ;; make-set: forest X -> void