洛谷P2853 [USACO06DEC]牛的野餐Cow Picnic

题目描述

The cows are having a picnic! Each of Farmer John's K (1 ≤ K ≤ 100) cows is grazing in one of N (1 ≤ N ≤ 1,000) pastures, conveniently numbered 1...N. The pastures are connected by M (1 ≤ M ≤ 10,000) one-way paths (no path connects a pasture to itself).

The cows want to gather in the same pasture for their picnic, but (because of the one-way paths) some cows may only be able to get to some pastures. Help the cows out by figuring out how many pastures are reachable by all cows, and hence are possible picnic locations.

K(1≤K≤100)只奶牛分散在N(1≤N≤1000)个牧场．现在她们要集中起来进餐．牧场之间有M(1≤M≤10000)条有向路连接，而且不存在起点和终点相同的有向路．她们进餐的地点必须是所有奶牛都可到达的地方．那么，有多少这样的牧场呢？

输入输出格式

输入格式：

Line 1: Three space-separated integers, respectively: K, N, and M

Lines 2..K+1: Line i+1 contains a single integer (1..N) which is the number of the pasture in which cow i is grazing.

Lines K+2..M+K+1: Each line contains two space-separated integers,
respectively A and B (both 1..N and A != B), representing a one-way path
from pasture A to pasture B.

输出格式：

Line 1: The single integer that is the number of pastures that are reachable by all cows via the one-way paths.

输入输出样例

输入样例#1：

2 4 4
2
3
1 2
1 4
2 3
3 4

输出样例#1：

2

说明

The cows can meet in pastures 3 or 4.

以每一头牛为起点，DFS标记所有能到达的点。如果一个点被标记的次数等于牛总数，那么将它计入答案。

 /*by SilverN*/
 #include<iostream>
 #include<algorithm>
 #include<cstring>
 #include<cstdio>
 #include<cmath>
 #include<vector>
 using namespace std;
 const int mxn=;
 int read(){
     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;
 }
 vector<int>e[mxn];
 int cnt[mxn];
 bool vis[mxn];
 void DFS(int u){
     vis[u]=;
     cnt[u]++;
     for(int i=;i<e[u].size();i++){
         int v=e[u][i];
         if(!vis[v]){
             DFS(v);
         }
     }
     return;
 }
 int K,n,m;
 int cow[mxn];
 int main(){
     int i,j,u,v;
     K=read();n=read();m=read();
     for(i=;i<=K;i++){
         cow[i]=read();
     }
     for(i=;i<=m;i++){
         u=read();v=read();
         e[u].push_back(v);
     }
     for(i=;i<=K;i++){
         memset(vis,,sizeof vis);
         DFS(cow[i]);
     }
     int ans=;
     for(i=;i<=n;i++)
         if(cnt[i]==K)ans++;
     cout<<ans<<endl;
     return ;
 }