[USACO09JAN]Earthquake Damage

嘟嘟嘟

刚开始因为没看到只能走没有损坏的农场，磨叽了20多分钟……不管了，写题解吧。

首先如果一个点不能到达原点，那么和他相邻的点也不能到达原点，所以刚开始我们把不能走的点和他相邻的点都打上标记，然后跑dfs就行了。

 #include<cstdio>
 #include<iostream>
 #include<algorithm>
 #include<cmath>
 #include<cstring>
 #include<cstdlib>
 #include<vector>
 #include<queue>
 #include<stack>
 #include<cctype>
 using namespace std;
 #define enter puts("")
 #define space putchar(' ')
 #define Mem(a) memset(a, 0, sizeof(a))
 typedef long long ll;
 typedef double db;
 const int INF = 0x3f3f3f3f;
 const db eps = 1e-;
 const int maxn = 3e4 + ;
 inline ll read()
 {
     ll ans = ;
     char ch = getchar(), last = ' ';
     while(!isdigit(ch)) last = ch, ch = getchar();
     while(isdigit(ch)) ans = (ans << ) + (ans << ) + ch - '', ch = getchar();
     if(last == '-') ans = -ans;
     return ans;
 }
 inline void write(ll x)
 {
     if(x < ) putchar('-'), x = -x;
     if(x >= ) write(x / );
     putchar(x %  + '');
 }

 int p, c, n;
 vector<int> v[maxn];
 bool vis[maxn];
 int ans;

 void dfs(int now)
 {
     vis[now] = ; ans--;
     for(int i = ; i < (int)v[now].size(); ++i)
         if(!vis[v[now][i]]) dfs(v[now][i]);
 }

 int main()
 {
     p = read(); c = read(); n = read();
     for(int i = ; i <= c; ++i)
     {
         int x = read(), y = read();
         v[x].push_back(y); v[y].push_back(x);
     }
     for(int i = ; i <= n; ++i)
     {
         int x = read(); vis[x] = ;
         for(int j = ; j < (int)v[x].size(); ++j) vis[v[x][j]] = ;
     }
     ans = p; dfs();
     write(ans); enter;
     return ;
 }