ICPC2004 ProblemB

Read and Black

(defun list-to-2d-array (list)
  (make-array (list (length list)
                    (length (first list)))
              :initial-contents list))

(defun read-map (width height)
  (list-to-2d-array
   (loop for i from 1 upto height
      collect (coerce (read-line) 'list))))

(defun find-start-pos (width height map)
  (let ((posX nil) (posY nil))
    (loop for y from 0 upto (1- height)
       do
	 (loop for x from 0 upto (1- width)
	    when (char= (aref map y x) #\@)
	    do
	      (setf posX x posY y)))
    (values posX posY)))

(defun reachable-tiles (x y width height map)
  (cond
    ((not (and (<= 0 x (1- width)) (<= 0 y (1- height))))
     0)
    ((char= #\# (aref map y x)) 0)
    (t (setf (aref map y x) #\#)
       (+  1
	   (reachable-tiles (1- x) y width height map)
	   (reachable-tiles (1+ x) y width height map)
	   (reachable-tiles x (1+ y) width height map)
	   (reachable-tiles x (1- y) width height map)))))

(defun main ()
  (loop
     for w = (read)
     for h = (read)
     until (and (zerop w ) (zerop h))
     do
       (let ((map (read-map w h)))
	 (multiple-value-bind (x y) (find-start-pos w h map)
	   (format t "~A~%" (reachable-tiles x y w h map))))))

再帰で綺麗に書けるのが良いですね。しかし、mapを直接変更してるのはすこし残念です。