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)

def dfs a,x,y
	a[x][y]="o" if a[x][y] != "o"
	dfs a,x-1,y if x-1 >= 0 && a[x-1][y] == "."
	dfs a,x+1,y if x+1 < a.length && a[x+1][y] == "."
	dfs a,x,y-1 if y-1 >= 0 && a[x][y-1] == "."
	dfs a,x,y+1 if y+1 < a[0].length && a[x][y+1] == "."
end

while true
	args = gets.split(" ").map{|e|e.to_i}
	break if args[0] == 0 && args[1] == 0
	a=[]
	start=[-1,-1]
	args[1].times{|i|
		a.push gets.chomp
		if a[i].index("@") != nil
			start=i,a[i].index("@")
		end
	}
	dfs a,start[0],start[1]
	count = 0
	a.each{|e|count+=e.count("o")}
	p count
end