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 Ruby (judge verdict: Accepted)

while true
    w, h = gets.chomp.split.map(&:to_i)
    break if [w, h] == [0, 0]

    map = []
    x, y = nil, nil
    h.times{|j|
        line = gets.chomp
        map << line
        if z = line.index("@")
            x, y = z, j
        end
    }
#    map.each{|m| puts m}
#    puts "x#{x} y#{y}"
    sum = 0
    list = [[x, y]]
    until list.empty?
        x2, y2 = list.pop
        if map[y2][x2] != "#"
            map[y2][x2] = "#"
            sum += 1
            list << [x2+1, y2] if x2 < w-1 && map[y2][x2+1] == "."
            list << [x2-1, y2] if x2 > 0 && map[y2][x2-1] == "."
            list << [x2, y2+1] if y2 < h-1 && map[y2+1][x2] == "."
            list << [x2, y2-1] if y2 > 0 && map[y2-1][x2] == "."
        end
    end
    puts sum
#    puts
end