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 {
  private static Scanner sc = new Scanner(System.in);
  public static void main(String...args) {
    while(sc.hasNext()) {
      final int W = sc.nextInt();
      final int H = sc.nextInt();
      if (W == 0 && H == 0)
        break;
      solve(W, H);
    }
  }
  private static void solve(final int W, final int H) {
    final char[][] map = new char[H][];
    for(int i = 0; i < H; i++)
      map[i] = sc.next().toCharArray();
    final Deque<Integer> deq = new ArrayDeque<Integer>();
    for(int i = 0; i < H; i++)
      for(int j = 0; j < W; j++)
        if(map[i][j] == '@')
          deq.offer(i * W + j);
    int ans = 0;
    while(!deq.isEmpty()) {
      ans++;
      final int p = deq.poll();
      final int i = p / W;
      final int j = p % W;
      for(int d = 0; d < 4; d++) {
        final int ni = i + di[d];
        final int nj = j + dj[d];
        if(0 <= ni && ni < H && 0 <= nj && nj < W && map[ni][nj] == '.') {
          map[ni][nj] = '#';
          deq.offer(ni * W + nj);
        }
      }
    }
    System.out.println(ans);
  }
  private static final int[] di = { 1, 0, -1, 0};
  private static final int[] dj = { 0, 1, 0, -1 };
}