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{
	static int w,h;
	static char map[][]=new char[25][25];
	static int dx[] = {0,1,0,-1};
	static int dy[] = {1,0,-1,0};
	static int[][] kioku =new int[25][25];
	static void dfs(int x,int y){
		if(x<0||x>=w||y<0||y>=h)return;
		if(kioku[y][x]!=0||map[y][x]=='#')return;
		kioku[y][x]=1;
		for(int i=0;i<4;i++){
			dfs(x+dx[i],y+dy[i]);
		}
		
	}

	public static void main(String[] args) {
		Scanner in = new Scanner(System.in);
		while(true){
			w=in.nextInt();
			h=in.nextInt();
			if(w+h==0)break;
			int sx=0,sy=0;
			for(int i=0;i<h;i++){
				String test = in.next();
				for(int s=0;s<w;s++){
					kioku[i][s]=0;
					map[i][s]=test.charAt(s);
					if(test.charAt(s)=='@'){
						sx=s;sy=i;
					}
				}
			}
			dfs(sx,sy);
			int result = 0;
			for(int i=0;i<h;i++){
				for(int s=0;s<w;s++){
					result += kioku[i][s];
				}
			}
			System.out.println(result);
		}
	}
}