tasks a-s

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

@@ -0,0 +1,22 @@
+(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 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 " ". "))))
+  (terpri))
+
+(let ((i 0) (n 8))
+  (mapc #'(lambda (s)
+	    (format t "Solution ~a:~%" (incf i))
+	    (show-solution s n))
+	(n-queens n n)))