解:

dfs部分:

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

完整代码:

#include<bits/stdc++.h>
using namespace std;
char a[55][55];
int n,m,sx,sy,ans = 0;
int dx[] = {0,-1,0,1},dy[] = {-1,0,1,0};
void dfs(int x,int y){
    for(int i = 0;i<4;i++){
        int nx = x+dx[i];
        int ny = y+dy[i];
        if(nx<1||nx>n||ny<1||ny>m) continue;
        if(a[nx][ny]!='.') continue;
        ans++;
        a[nx][ny] = '#';
        dfs(nx,ny);
    }
}
int main(){
	cin>>n>>m;
    for(int i = 1;i<=n;i++){
        for(int j = 1;j<=m;j++){
            cin>>a[i][j];
            if(a[i][j] == '@'){
                sx = i,sy = j;
            }
        }
    }
    a[sx][sy] = '#';
    ans++;
    dfs(sx,sy);
    cout<<ans;
	return 0;
}