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 H,W;
	static int f[][];
	static int dx[]={1,-1,0,0};
	static int dy[]={0,0,1,-1};
	static int cnt;
	public static void main(String[] args) 
	{
		Scanner in=new Scanner(System.in);
		for(;;)
		{
			W=in.nextInt();
			H=in.nextInt();
			if((W|H)==0)
				return;
			int sw=-1;
			int sh=-1;
			f=new int[H][W];
			cnt=0;
			for(int i=0;i<H;i++)
			{
				String s=in.next();
				for(int j=0;j<W;j++)
				{
					if(s.charAt(j)=='#')
						f[i][j]=1;
					else
						f[i][j]=0;

					if(s.charAt(j)=='@')
					{
						sw=j;
						sh=i;
					}
				}
			}
			System.out.println(dfs(sw,sh));
		}
	}

	static  int dfs(int w,int h)
	{
		cnt++;
		f[h][w]=1;
		for(int i=0;i<4;i++)
		{
			int nw=dx[i]+w;
			int nh=dy[i]+h;
			if(nw>=0&&nw<W&&nh>=0&&nh<H&&f[nh][nw]==0)
				dfs(nw,nh);
		}
		return cnt;
	}
	static public void debug(Object... o)
	{
		System.out.println(Arrays.deepToString(o));
	}
}