cf 525D.Arthur and Walls

判断2*2的正方形，是不是3个"."1个"*"然后暴力bfs就好。（这种处理也是挺神奇的2333%%题解）

 #include<bits/stdc++.h>
 #define INF 0x7fffffff
 #define LL long long
 #define N 100005
 #define pill pair<int ,int >
 using namespace std;
 inline int ra()
 {
     int x=,f=; char ch=getchar();
     while (ch<'' || ch>'') {if (ch=='-') f=-; ch=getchar();}
     while (ch>='' && ch<='') {x=x*+ch-''; ch=getchar();}
     return x*f;
 }
 const int dir_x[][] = {{,,},{,-,-},{-,-,},{,,}};
 const int dir_y[][] = {{,,},{,,},{,-,-},{,-,-}};
 char mat[][];
 int n,m;
 void bfs()
 {
     queue<pair<int ,int > > q;
     for (int i=; i<n; i++)
         for (int j=; j<m; j++)
             if (mat[i][j]=='.') q.push(make_pair(i,j));
     while (!q.empty())
     {
         pair<int , int > now=q.front(); q.pop();
         int x=now.first,y=now.second;
         int pos_x,pos_y,cnt;
         for (int i=; i<; i++)
         {
             cnt=;
             int ok=;
             for (int j=; j<; j++)
             {
                 int xx=x+dir_x[i][j];
                 int yy=y+dir_y[i][j];
                 if (xx>= && xx<n && yy>= && yy<m)
                 {
                     if (mat[xx][yy]=='*')
                     {
                         pos_x=xx; pos_y=yy;
                         cnt++;
                     }
                 }
                 else {
                     ok=;
                     break;
                 }
             }
             if (ok && cnt==)
             {
                 mat[pos_x][pos_y]='.';
                 q.push(make_pair(pos_x,pos_y));
             }
         }
     }
     return;
 }

 void print()
 {
     for (int i=; i<n; i++)
         puts(mat[i]);
 }
 int main()
 {
     scanf("%d%d",&n,&m);
     for (int i=; i<n; i++)
         scanf("%s",mat[i]);
     bfs();
     print();
     return ;
 }