CLisp: NQueen

Author: minmaxmean

BackTracking/NQueens/nqueen.lisp

@@ -0,0 +1,40 @@
+(setq N 6)
+(setq rows  (make-array N))
+(setq cols  (make-array N))
+(setq diag1 (make-array (+ N N 1)))
+(setq diag2 (make-array (+ N N 1)))
+
+(defun print-board()
+  (format t "~%")
+  (loop for i from 0 to (- N 1) do
+    (format t "~%")
+    (loop for j from 0 to (- N 1) do
+      (cond
+        ((eq (aref rows i) j) (prin1 'Q))
+        (t (prin1 '-))
+      )
+    )
+  )
+)
+
+(defun rec(j)
+  (when (>= j N) (print-board) (return-from rec))
+  (loop for i from 0 to (- N 1) do
+    (let ((x (+ (- i j 1) N)) (y (- (+ N N) i j)))
+      (when (and (null (aref rows i)) (null (aref diag1 x)) (null (aref diag2 y)))
+        (setf (aref cols j) j)
+        (setf (aref rows i) j)
+        (setf (aref diag1 x) j)
+        (setf (aref diag2 y) j)
+        (rec (+ j 1))
+        (setf (aref cols j) nil)
+        (setf (aref rows i) nil)
+        (setf (aref diag1 x) nil)
+        (setf (aref diag2 y) nil)
+      )
+    )
+  )
+)
+
+; (print-board)
+(rec 0)