速さが欲しかった実装。 http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=1130&lang=jp

RedAndBlack.c

#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>


struct {
	int_fast32_t x, y;
} typedef Position;


int main(){
	const Position cands[] = {{1, 0}, {-1, 0}, {0, 1}, {0, -1}};
	uint_fast32_t w, h, x, y, stack_itr, count;
	uint_fast8_t c, i;
	Position p;

	while(scanf("%lu %lu\n", &w, &h) && w){
		bool tile[h][w];
		Position stack_data[h*w];
		stack_itr = 1;
		count = 0;

		for(y=0; y<h; ++y){
			for(x=0; x<w; ++x){
				c = fgetc(stdin);

				tile[y][x] = c == '.';

				if(c == '@'){
					stack_data[0].x = x;
					stack_data[0].y = y;
				}
			}
			fgetc(stdin);
		}

		while(stack_itr){
			count++;
			stack_itr--;

			p = stack_data[stack_itr];

			for(i=0; i<4; ++i){
				x = p.x + cands[i].x;
				y = p.y + cands[i].y;

				if(x < w && y < h && tile[y][x]){
					tile[y][x] = false;
					stack_data[stack_itr].x = x;
					stack_data[stack_itr].y = y;
					stack_itr++;
				}
			}
		}

		printf("%lu\n", count);
	}

	return 0;
}