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: Wrong Answer)

import java.util.Scanner;
import java.util.Stack;

public class Main {
	static int[] v1 = { 0, 1, 0, -1 };
	static int[] v2 = { 1, 0, -1, 0 };
	static int[][] field;
	static int count = 0;
	public static void main(String[] args) {

		Scanner cin = new Scanner(System.in);
		while (true) {
			int w = cin.nextInt();
			int h = cin.nextInt();
			if(w+h==0){break;}
			
			int startx = 0, starty = 0;
			count=0;
			field = new int[h + 2][w + 2];
			for (int i = 1; i <= h; i++) {
				String str = cin.next();
				for (int j = 1; j <= w; j++) {
					if (str.charAt(j-1) == '.') {
						field[i][j] = 1;
					} else if (str.charAt(j-1) == '#') {
						field[i][j] = 2;
					} else if (str.charAt(j-1) == '@') {
						field[i][j] = 1;
						startx = i;
						starty = j;
					}
				}
			}
			Integer[] start = {startx,starty};
			Stack<Integer[]> s = new Stack<Integer[]>();
			s.add(start);
			while(!s.isEmpty()){
				Integer[] a = s.pop().clone();
				count++;
				field[a[0]][a[1]]=2;
				for(int i = 0;i<4;i++){
					int xx = a[0]+v1[i];
					int yy = a[1]+v2[i];
					if(field[xx][yy]==1){
						Integer[] next={xx,yy};
						s.add(next.clone());
					}
				}
			}
			System.out.println(count);
		}
	}

}