dfs找一下 (26ms)

#include <iostream>
#include <cstring>
#include <cstdio>
#include <algorithm>
using namespace std;
const int N = 25;
int n,m;
char g[N][N];
int dx[4] = {-1,0,1,0},dy[4] = {0,1,0,-1};

int dfs(int x,int y){
    int res = 1;
    g[x][y] = '#';
    for(int i = 0;i < 4;++i){
        int a = x + dx[i],b = y + dy[i];
        if(a < 0 || a >= n || b < 0 || b >= m || g[a][b] != '.')    continue;
        res += dfs(a,b);
    }
    return res;
}

int main(){
    while(scanf("%d%d",&m,&n),n || m){
        for(int i = 0;i < n;++i)    scanf("%s",g[i]);
        int x,y;
        for(int i = 0;i < n;++i){
            for(int j = 0;j < m;++j){
                if(g[i][j] == '@')  x = i,y = j;
            }
        }
        printf("%d\n",dfs(x,y));
    }
    return 0;
}

bfs做法 (44ms)

#include <iostream>
#include <cstring>
#include <cstdio>
#include <algorithm>
#include <queue>
#define x first
#define y second
using namespace std;
typedef pair<int, int> PII;
const int N = 25;
int n,m;
char g[N][N];
int dx[4] = {-1,0,1,0},dy[4] = {0,1,0,-1};

int bfs(int sx,int sy){
    queue<PII> q;
    q.push({sx,sy});
    g[sx][sy] = '#';
    int res = 0;
    while(q.size()){
        auto t = q.front();
        q.pop();
        res++;
        for(int i = 0;i < 4;++i){
            int x = t.x + dx[i],y = t.y + dy[i];
            if(x < 0 || x >= n || y < 0 || y >= m || g[x][y] != '.')    continue;
            g[x][y] = '#';
            q.push({x,y});
        }
    }
    return res;
}

int main(){
    while(scanf("%d%d",&m,&n),n || m){
        for(int i = 0;i < n;++i)    scanf("%s",g[i]);
        int x,y;
        for(int i = 0;i < n;++i){
            for(int j = 0;j < m;++j){
                if(g[i][j] == '@')  x = i,y = j;
            }
        }
        printf("%d\n",bfs(x,y));
    }
    return 0;
}