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 Python3 (judge verdict: Accepted)

from collections import deque
while 1:
    W,H = map(int,input().split())
    if W==0 and H==0:break
    tizu = []
    for i in range(H):
        row = input()
        if "@" in row:
            start = (i,row.index("@"))#h,w の順
        tizu.append(row)
    went = [[False]*W for i in range(H)]
    went[start[0]][start[1]] = True
    ans = 1
    queue = deque()#append,popleft
    queue.append(start)
    while queue:
        curh,curw = queue.popleft()
        for dh,dw in ((0,1),(0,-1),(1,0),(-1,0)):
            newh = curh + dh
            neww = curw + dw
            if 0<=newh<H and 0<=neww<W:
                if tizu[newh][neww] == "." and not went[newh][neww]:
                    went[newh][neww] =True
                    queue.append((newh,neww))
                    ans += 1
    print(ans)