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.*;

class Main {
    static Scanner sc = new Scanner(System.in);
    static int w,h;
    static char[][] map;
    static int[] dx = {-1, 0, 0, 1};
    static int[] dy = { 0,-1, 1, 0}; 
    
    
    public static void main(String[] args){
        while(true){
            w = sc.nextInt();
            h = sc.nextInt();
            if(w==0&&h==0)break;
            
            map = new char[h][w];
            
            for(int i=0;i<h;i++){
                map[i] = sc.next().toCharArray();
            }
            loop:
            for(int i=0;i<h;i++){
                for(int j=0;j<w;j++){
                    if(map[i][j]=='@'){
                        dfs(j,i);
                        break loop;
                    }
                }
            }
            
            int ans=0;
            for(int i=0;i<h;i++){
                for(int j=0;j<w;j++){
                    if(map[i][j]=='*'){
                        ans++;
                    }
                }
            }
            
            System.out.println(ans);
            
        }
        
    }
    
    static void dfs(int x,int y){
        map[y][x]='*';
        for(int i=0;i<4;i++){
            int nx = x + dx[i];
            int ny = y + dy[i];
            
            if(nx>=0 && nx<w && ny>=0 && ny<h && map[ny][nx]=='.'){
                dfs(nx,ny);
            }
        }
    }
}