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 search(a, x, y, w, h, count)
  if x+1 < w and a[y][x+1] != nil and a[y][x+1] == '.' and not count.include? [x+1,y]
    count << [x+1,y]
    search(a, x+1, y, w, h, count)
  end
  if x-1 >= 0 and a[y][x-1] != nil and a[y][x-1] == '.' and not count.include? [x-1,y]
    count << [x-1,y]
    search(a, x-1, y, w, h, count)
  end
  if y+1 < h and a[y+1][x] != nil and a[y+1][x] == '.' and not count.include? [x,y+1]
    count << [x,y+1]
    search(a, x, y+1, w, h, count)
  end
  if y-1 >= 0 and a[y-1][x] != nil and a[y-1][x] == '.' and not count.include? [x,y-1]
    count << [x,y-1]
    search(a, x, y-1, w, h, count)
  end
end

loop do
  w, h = gets.split.map(&:to_i)
  exit if w == 0 and h == 0
  a = h.times.map.with_index{gets.chomp.chars}

  x = y = 0
  y = a.find_index{|a| a.index("@")}
  x = a[y].index("@")

  count = []
  search(a, x, y, w, h, count)
  puts (count.length + 1)
end