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 Python (judge verdict: Accepted)

import sys
import collections

x = [1, 0, -1, 0]
y = [0, 1, 0, -1]
while True:
    W, H = map(int, sys.stdin.readline().split())
    if W == 0:
        break
    
    tile = [[0 for i in xrange(W + 2)] for j in xrange(H + 2)]
    
    visited = set()
    next = set()
    
    for i in xrange(H):
        line = raw_input().strip()
        for j in xrange(W):
            if line[j] == '.':
                tile[i + 1][j + 1] = 1
            if line[j] == '@':
                tile[i + 1][j + 1] = 1
                next.add((i + 1, j + 1))
#    for line in tile:
#        print line
    while len(next) > 0:
        temp_next = set()
        for n in next:
            visited.add(n)
            for i in xrange(4):
                temp = (n[0] + x[i], n[1] + y[i])
                if temp not in visited and tile[temp[0]][temp[1]] == 1:
                    temp_next.add(temp)
        next = temp_next
    print len(visited)