df2880a47218

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/problems/sign.lisp	Mon Dec 24 00:41:30 2018 -0500
@@ -0,0 +1,37 @@
+(in-package :rosalind)
+
+(defparameter *input-sign* "2")
+
+(defparameter *output-sign* "8
+-1 -2
+-1 2
+1 -2
+1 2
+-2 -1
+-2 1
+2 -1
+2 1")
+
+
+(defun sign-permutations (list)
+  (if (null list)
+    (list '()) ;; there is exactly one permutation of the empty list: ()
+    (destructuring-bind (n . more) list
+      (append (mapcar (curry #'cons n) (sign-permutations more))
+              (mapcar (curry #'cons (- n)) (sign-permutations more))))))
+
+(define-problem sign (data string)
+    *input-sign*
+    *output-sign*
+  (let* ((n (parse-integer data))
+         (count (* (expt 2 n)
+                   (factorial n)))
+         (perms (mapcan #'sign-permutations
+                        (permutations (alexandria:iota n :start 1)))))
+    ;; sort to ensure consistent output for the unit test
+    (format nil "~D~%~{~A~^~%~}"
+            count
+            (sort (mapcar (curry #'str:join " ") perms) #'string<))))
+
+;; (problem-sign "2")
+;; (solve sign)