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 cx, cy, lines, visited
  return 0 if visited.include? [cx, cy]
  visited << [cx, cy]

  # p [cx, cy]
  # p [lines[cy][cx + 1], lines[cy][cx - 1], lines[cy + 1][cx], lines[cy - 1][cx]]
  res = 1
  res += dfs cx + 1, cy, lines, visited if lines[cy][cx + 1] == "."
  res += dfs cx - 1, cy, lines, visited if lines[cy][cx - 1] == "."
  res += dfs cx, cy + 1, lines, visited if lines[cy + 1][cx] == "."
  res += dfs cx, cy - 1, lines, visited if lines[cy - 1][cx] == "."

  res
end

loop do
  w, h = gets.split.map(&:to_i)
  break if w == 0
  x = 0
  y = 0
  lines = [["#"] * (w + 2)] # ??????
  1.upto(h) do |i|
    line = "##{gets.chomp}#".split "" # ??????
    idx = line.index("@")
    if idx
      line[idx] = "."
      x = idx
      y = i
    end
    lines << line
  end
  lines << ["#"] * (w + 2) # ??????

  # require "pp"
  # pp lines
  # p [[x, y]]
  puts dfs x, y, lines, []
end