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.*;

class Main{
   public static void main(String a[]) throws java.io.IOException{
       Scanner scan =new Scanner (System.in);
       Pair pair= new Pair();
       Queue<Pair> fifo = new LinkedList<Pair>();

       while(true){

           int W =scan.nextInt();
           int H =scan.nextInt();
           if((W|H)==0)break;
           char  [][] map = new char [H][W];
           for(int i=0;i<H;i++){
               String  tmp =scan.next();
               for(int j=0;j<W;j++){
                   map[i][j] = tmp.charAt(j);
                   if(tmp.charAt(j)=='@'){
                       
                       pair.x=j;
                       pair.y=i;
                       fifo.add(pair);
                   }
               }
           }
          int count =0;
          while(fifo.peek()!=null){  
           pair = fifo.poll();
           int x=pair.x;
           int y=pair.y;
           int[] dx ={0,1,0,-1};
           int[] dy ={1,0,-1,0};
          for(int i=0;i<4;i++){
              int nx =x+dx[i];
              int ny =y+dy[i];
              if(0<=nx&&nx<W&&0<=ny&&ny<H&&map[ny][nx]!='#'){
                  Pair npair =new Pair();
                  npair.x=nx;
                  npair.y=ny;
                  fifo.add(npair);
                  map[ny][nx] = '#';
                  count++;
              }
          } 
          }
          System.out.println(count);

       
       }
}
}

class Pair{
    public int x;
    public int y;
}