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.Scanner;

public class Main {
	int H, W;
	int[] dx = {1, 0, -1, 0};
	int[] dy = {0, 1, 0, -1};
	void run(){
		Scanner sc = new Scanner(System.in);
		while(true){
			W = sc.nextInt();
			H = sc.nextInt();
			if((H | W) == 0){
				break;
			}
			sc.nextLine();
			char[][] field = new char[H][W];
			for(int i = 0; i < H; ++i){
				field[i] = sc.nextLine().toCharArray(); 
			}
			for(int i = 0; i < H; ++i){
				for(int j = 0; j < W; ++j){
					if(field[i][j] == '@'){
						System.out.println(dfs(field, i, j));
					}
				}
			}
		}
	}
	int dfs(char[][] field, int y, int x){
		if(y < 0 || H <= y || x < 0 || W <= x){
			return 0;
		}
		if(field[y][x] == '#'){
			return 0;
		}
		field[y][x] = '#';
		int ret = 0;
		for(int i = 0; i < 4; ++i){
			ret += dfs(field, y+dy[i], x+dx[i]);
		}
		return ret+1;
	}
	public static void main(String[] args) {
		new Main().run();
	}
}