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

		new A1130().ans(sc);
	}

}

class Code {

}

class A1130 {

	int[] xdis = { -1, 0, +1, 0 };
	int[] ydis = { 0, -1, 0, +1 };

	int[][] cm;

	public void ans(Scanner sc) {
		while (true) {
			String str = sc.nextLine();
			String[] cut = str.split(" ");
			int W = Integer.parseInt(cut[0], 10);
			int H = Integer.parseInt(cut[1], 10);

			if (W == 0) {
				break;
			}

			cm = new int[H][W];

			int x = 0;
			int y = 0;
			for (int h = 0; h < H; h++) {
				String map = sc.nextLine();
				for (int w = 0; w < W; w++) {
					char c = map.charAt(w);
					switch (c) {
					case '.':
						cm[h][w] = 0;
						break;
					case '@':
						x = w;
						y = h;
						cm[h][w] = 0;
						break;
					case '#':
						cm[h][w] = -1;
						break;
					}
				}
			}
			rec(x, y);


			int count=0;
			for (int h = 0; h < H; h++) {
				for (int w = 0; w < W; w++) {
					if(cm[h][w]==1){
						count++;
					}
				}
			}
			System.out.println(count);

		}
	}

	void rec(int x, int y) {
		if (y < 0 || cm.length <= y) {
			return;
		}
		if (x < 0 || cm[0].length <= x) {
			return;
		}
		if (cm[y][x] != 0) {
			return;
		}

		cm[y][x] = 1;
		for (int i = 0; i < xdis.length; i++) {
			rec(x + xdis[i], y + ydis[i]);
		}
	}
}