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;

class Main
{
	private static StringBuffer map[];
	private static int count;
	static int[] dx = {0, 1, 0 ,-1};
	static int[] dy = {-1, 0, 1, 0};
	public static void main(String args[])
	{
		Scanner sc = new Scanner(System.in);
		while (true)
		{
			int n = sc.nextInt(), m = sc.nextInt();
			if (n == 0 && m == 0) break;
			map = new StringBuffer[m];
			count = 0;
			int x = 0, y = 0;
			for (int i = 0; i < m; i++)
			{
				String str = sc.next();
				if (str.indexOf('@') != -1)
				{
					x = str.indexOf('@');
					y = i;
				}
				map[i] = new StringBuffer(str);
			}
			walk(x, y);
			System.out.println(count);
		}
	}
	private static void walk(int x, int y)
	{
		count++;
		map[y].setCharAt(x, '#');
		for (int i = 0; i < 4; i++)
		{
			try{
				if (map[y+dy[i]].charAt(x+dx[i]) == '.') walk(x+dx[i], y+dy[i]);
			}catch (IndexOutOfBoundsException e){}
		}
	}
}