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)

require 'pp'

while (w, h = gets.chomp.split(" ").map{|str| str.to_i}) != [0, 0]
	queue = []
	table = []
	for y in 0..(h - 1)
		table << gets.chomp.split("").map.with_index do |char, x|
			if char == "."
				0
			elsif char == "#"
				-1
			elsif char == "@"
				queue << [y, x]
				0
			end
		end
	end
	while !queue.empty?
		y, x = queue.shift
		if table[y][x] == 0
			table[y][x] = 1
			add = []
			add << [y, x - 1] if x != 0 
			add << [y - 1, x] if y != 0
			add << [y, x + 1] if x != w - 1
			add << [y + 1, x] if y != h - 1
			queue += add
		end
	end
	puts table.flatten.count(1)
end