Just another update

Task/N-queens-problem/Common-Lisp/n-queens-problem-1.lisp

@@ -1,22 +1,19 @@
-(defun n-queens (n m)
-  (if (= n 1)
-    (loop for x from 1 to m collect (list x))
-    (loop for sol in (n-queens (1- n) m) nconc
-	  (loop for col from 1 to m when
-		(loop for row from 0 to (length sol) for c in sol
-		      always (and (/= col c)
-				  (/= (abs (- c col)) (1+ row)))
-		      finally (return (cons col sol)))
-		collect it))))
+(defun queens (n &optional (m n))
+  (if (zerop n)
+      (list nil)
+      (loop for solution in (queens (1- n) m)
+            nconc (loop for new-col from 1 to m
+                         when (loop for row from 1 to n
+                                     for col in solution
+                                     always (/= new-col col (+ col row) (- col row)))
+                         collect (cons new-col solution)))))
 
-(defun show-solution (b n)
-  (loop for i in b do
-	(format t "~{~A~^~}~%"
-		(loop for x from 1 to n collect (if (= x i) "Q " ". "))))
+(defun print-solution (solution)
+  (loop for queen-col in solution
+        do (loop for col from 1 to (length solution)
+                  do (write-char (if (= col queen-col) #\Q #\.)))
+           (terpri))
   (terpri))
 
-(let ((i 0) (n 8))
-  (mapc #'(lambda (s)
-	    (format t "Solution ~a:~%" (incf i))
-	    (show-solution s n))
-	(n-queens n n)))
+(defun print-queens (n)
+  (mapc #'print-solution (queens n)))