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.Arrays;
import java.util.Scanner;

public class Main {
  Scanner sc;

  boolean[][] done;
  char[][] field;

  int[][] ofs = new int[][] { { 0, 1 }, { 1, 0 }, { -1, 0 }, { 0, -1 } };

  int dfs(int x, int y) {
    if ( done[ y ][ x ] || field[ y ][ x ] == '#' )
      return 0;

    int sum = 1;
    done[ y ][ x ] = true;
    for ( int[] d : ofs ) {
      sum += dfs( x + d[ 0 ], y + d[ 1 ] );
    }

    return sum;
  }

  void run() {
    for ( ;; ) {
      int w, h;
      w = ni();
      h = ni();

      if ( ( w | h ) == 0 ) {
        break;
      }

      done = new boolean[h + 2][w + 2];
      field = new char[h + 2][w + 2];

      for ( int i = 0; i < h + 2; ++i ) {
        for ( int j = 0; j < w + 2; ++j ) {
          field[ i ][ j ] = '#';
        }
      }

      int sx = 0;
      int sy = 0;
      for ( int i = 1; i <= h; ++i ) {
        char[] str = ns().toCharArray();
        for ( int j = 1; j <= w; ++j ) {
          field[ i ][ j ] = str[ j - 1 ];
          if ( field[ i ][ j ] == '@' ) {
            sx = j;
            sy = i;
          }
        }
      }

      System.out.println( dfs( sx, sy ) );
    }
  }

  Main() {
    sc = new Scanner( System.in );
  }

  int ni() {
    return sc.nextInt();
  }

  String ns() {
    return sc.next();
  }

  public static void main(String[] args) {
    new Main().run();
  }

  void debug(Object... os) {
    System.err.println( Arrays.deepToString( os ) );
  }
}