这道题,非常困难,写不出来👍

#include <iostream>
#include <cstring>
using namespace std;
char mp[21][21];
int n,m,x,y,ans = 1,vis[21][21];//vis数组记录,以防重复搜索
int dx[4] = {0,1,0,-1},dy[4] = {1,0,-1,0};
//四个方向
bool check(int a,int b)
{
   if(a > 0 && a <= m && b > 0 && b <= n) return true;
   return false;
}
//判断是否越界
void dfs(int a,int b)
{
   for(int i = 0;i < 4;i ++)//向四个方向进攻!
   {
      int xx = a + dx[i],yy = b + dy[i];
      if(check(xx,yy) && vis[xx][yy] == 0 && mp[xx][yy] == '.')
      {
         ans ++;
         vis[xx][yy] = 1;
         dfs(xx,yy);
      }
   }
}
int main()
{
   ios::sync_with_stdio(0);
   cin.tie(0),cout.tie(0);
   //无法编译
   cin >> n >> m;
   do
   {
      ans = 1;
      memset(vis,0,sizeof(vis));
      memset(mp,0,sizeof(mp));
      for(int i = 1;i <= m;i ++)
      {
         for(int j = 1;j <= n;j ++)
         {
            cin >> mp[i][j];
            if(mp[i][j] == '@') 
            {
               vis[i][j] = 1;
               x = i,y = j;
            }
            //记录起点
         }
      }
      dfs(x,y);
      cout << ans << "\n";
      cin >> n >> m;
   }while(n && m);
}