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.*;
import java.io.*;
import java.awt.geom.*;
import java.math.*;

public class Main {

	static final Scanner in = new Scanner(System.in);
	static final PrintWriter out = new PrintWriter(System.out,false);
	static boolean debug = false;

	static boolean solve() {
		int w = in.nextInt();
		int h = in.nextInt();
		if (w + h == 0) return false;

		int sy = -1, sx = -1;

		char[][] table = new char[h][w];
		for (int i=0; i<h; i++) {
			table[i] = in.next().toCharArray();
			for (int j=0; j<w; j++) {
				if (table[i][j] == '@') {
					sy = i; sx = j;
				}
			}
		}

		int[] dx = { 0, 1, 0,-1};
		int[] dy = { 1, 0,-1, 0};

		ArrayDeque<Integer> que = new ArrayDeque<>();
		int cnt = 0;
		que.add(sy*w+sx);

		boolean[][] visited = new boolean[h][w];
		visited[sy][sx] = true;

		while (que.size() > 0) {
			int c = que.pollFirst();
			cnt++;
			int cy = c/w, cx = c%w;
			for (int k=0; k<4; k++) {
				int ny = cy + dy[k], nx = cx + dx[k];
				if (ny < 0 || nx < 0 || h <= ny || w <= nx) continue;
				if (table[ny][nx] == '#' || visited[ny][nx]) continue;
				visited[ny][nx] = true;
				que.add(ny*w+nx);
			}
		}

		out.println(cnt);


		return true;
	}

	public static void main(String[] args) {
		debug = args.length > 0;
		long start = System.nanoTime();

		while(solve());
		out.flush();

		long end = System.nanoTime();
		dump((end - start) / 1000000 + " ms");
		in.close();
		out.close();
	}

	static void dump(Object... o) { if (debug) System.err.println(Arrays.deepToString(o)); }
}