optimize flatten function

diff --git a/lists.lisp b/lists.lisp
index 5cd193cefe96910174ef1c9a9a5039c0b1c9b7b5..eaa3f9d372aa601d079d6f24ead4798bc17ea467 100644 (file)
--- a/lists.lisp
+++ b/lists.lisp
@@ -73,11 +73,14 @@
             it
             nil)))))
 
-(defun flatten (lis)
-  (cond ((atom lis) lis)
-        ((listp (car lis))
-         (append (flatten (car lis)) (flatten (cdr lis))))
-        (t (append (list (car lis)) (flatten (cdr lis))))))
+(defun flatten (tree)
+  (let ((result '()))
+    (labels ((scan (item)
+               (if (listp item)
+                 (map nil #'scan item)
+                 (push item result))))
+      (scan tree))
+    (nreverse result)))
 
 ;;; Keyword functions