Problem:
Problem B: Red and Black

There is a rectangular room, covered with square tiles. Each tile is
colored either red or black. A man is standing on a black tile.
From a tile, he can move to one of four adjacent tiles. But he can't
move on red tiles, he can move only on black tiles.

Write a program to count the number of black tiles which he can reach
by repeating the moves described above.

Input

The input consists of multiple data sets.
A data set starts with a line containing two positive integers W and H;
W and H are the numbers of tiles in the x-
and y- directions, respectively. W and H are
not more than 20.

There are H more lines in the data set, each of which
includes W characters. Each character represents the color of a
tile as follows.

'.' - a black tile

'#' - a red tile

'@' - a man on a black tile(appears exactly once in a data set)

The end of the input is indicated by a line consisting of two zeros.

Output

For each data set, your program
should output a line which contains the number of
tiles he can reach from the initial tile (including itself).

Sample Input

6 9
....#.
.....#
......
......
......
......
......
#@...#
.#..#.
11 9
.#.........
.#.#######.
.#.#.....#.
.#.#.###.#.
.#.#..@#.#.
.#.#####.#.
.#.......#.
.#########.
...........
11 6
..#..#..#..
..#..#..#..
..#..#..###
..#..#..#@.
..#..#..#..
..#..#..#..
7 7
..#.#..
..#.#..
###.###
...@...
###.###
..#.#..
..#.#..
0 0

Output for the Sample Input

45
59
6
13

Solution in JAVA (judge verdict: Accepted)

import java.util.*;

public class Main {
	Scanner in = new Scanner(System.in);
	public static void main(String[] args) {
		new Main();
	}
	public Main(){
		new AOJ1130().doIt();
	}
	
	class AOJ1130{
		
		int w,h;
		char[][] map;
		int sx[] = {1,0,-1,0};
		int sy[] = {0,1,0,-1};
		void dfs(int x,int y){
			map[y][x] = '*';
			for(int i=0;i<4;i++){
				int nx = x+sx[i];
				int ny = y+sy[i];
				if(nx<0||nx>=w||ny<0||ny>=h)continue;
				if(map[ny][nx]=='#'||map[ny][nx]=='*')continue;
				dfs(nx,ny);
			}
		}
		
		void solve(){
			int startx=0,starty=0;
			for(int i=0;i<h;i++)for(int s=0;s<w;s++)if(map[i][s]=='@'){
				startx = s;starty = i;break;
			}
			dfs(startx,starty);
			int cnt = 0;
			for(int i=0;i<h;i++)for(int s=0;s<w;s++)if(map[i][s]=='*')cnt++;
			System.out.println(cnt);
		}
		
		void doIt(){
			while(in.hasNext()){
				w = in.nextInt();
				h = in.nextInt();
				if(w+h==0)break;
				map = new char[h][w];
				for(int i=0;i<h;i++)map[i] = in.next().toCharArray();
				solve();
			}
		}
	}
	
}