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 W, H;
	static char[][] tile;

	static int move(int x, int y) {
		int count = 1;

		if (y + 1 < H) {
			if (tile[y + 1][x] == '.') {
				tile[y + 1][x] = 'o';
				count += move(x, y + 1);
			}
		}
		if (y - 1 >= 0) {
			if (tile[y - 1][x] == '.') {
				tile[y - 1][x] = 'o';
				count += move(x, y - 1);
			}
		}
		if (x + 1 < W) {
			if (tile[y][x + 1] == '.') {
				tile[y][x + 1] = 'o';
				count += move(x + 1, y);
			}
		}
		if (x - 1 >= 0) {
			if (tile[y][x - 1] == '.') {
				tile[y][x - 1] = 'o';
				count += move(x - 1, y);
			}
		}

		return count;
	}

	public static void main(String[] args) {
		String temp;
		int atX = 0, atY = 0;
		int num;
		Scanner s = new Scanner(System.in);

		while (true) {
			W = s.nextInt();
			H = s.nextInt();
			
			if(W==0 && H==0) break;

			tile = new char[H][W];

			for (int i = 0; i < H; i++) {
				temp = s.next();
				tile[i] = temp.toCharArray();

				for (int j = 0; j < W; j++) {
					if (tile[i][j] == '@') {
						atX = j;
						atY = i;
					}
				}
			}

			num = move(atX, atY);

			System.out.println(num);
		}
		s.close();
	}

}