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{
	public static char[][] map;
	public static int[] dx = {0,0,1,-1};
	public static int[] dy = {1,-1,0,0};

	public static int dfs(int x, int y){
		int ans=1;
		map[y][x] = '#';
		for(int i=0;i<4;i++){
			int nx = x + dx[i];
			int ny = y + dy[i];
			if(0<=nx && nx<map[0].length && 0<=ny && ny<map.length && map[ny][nx] == '.'){
				ans += dfs(nx,ny);
			}
		}
		return ans;
	}

	public static void main(String[] args){
		Scanner sc= new Scanner(System.in);

		while(true){
			int w=sc.nextInt(), h=sc.nextInt();
			if(w==0 && h==0) break;

			map = new char[h][w];
			int x=0, y=0;
			for(int i=0;i<h;i++){
				map[i] = sc.next().toCharArray();
				for(int j=0;j<w;j++){
					if(map[i][j]=='@'){x=j; y=i; map[i][j]='.';}
				}
			}
			System.out.println(dfs(x,y));
		}
	}
}