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 C (judge verdict: Time Limit Exceeded)

#include <stdio.h>

char s[21][21];
int count, h, w;
int dx[] = { 0, 0, 1, -1 };
int dy[] = { 1, -1, 0, 0 };

void dfs(int x, int y) {
	int i;
	if (s[y][x] == '.' || s[y][x] == '@') {
		count++;
		s[y][x] = '#';
		for (i = 0; i < 4; i++) {
			dfs(x + dx[i], y + dy[i]);
		}
	}
	return 0;
}

int main(void) {
	char a;
	int i, j, x, y;
	while(1){
		scanf("%d %d", &w, &h);
		scanf("%c", &a);
		if (w == 0 && h == 0) {
			break;
		}

		for (i = 0; i<21; i++) {
			for (j = 0; j<21; j++) {
				s[i][j] = '#';
			}
		}
		count = 0;

		for (i = 1; i<h; i++) {
			for (j = 1; j<w; j++) {
				scanf("%c", &s[i][j]);
				if (s[i][j] == '@') {
					x = j;
					y = i;
				}
			}
			scanf("%c", &a);
		}
		dfs(x, y);
		printf("%d\n", count);
	}
	return 0;
}