P2853 [USACO06DEC]Cow Picnic S

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 #1

输出 #1

说明/提示

The cows can meet in pastures 3 or 4.

思路：从奶牛所在牧场出发，dfs遍历所有牧场，当牧场被遍历的次数==奶牛数时，表明所有奶牛都可到达此牧场，满足条件，ans++

代码如下：
#include<bits/stdc++.h>
using namespace std;
bool vis[1010];
int bian[1010],a[1010];
int k,n,m,ans;
vector<int> G[1010];
void dfs(int x){
   vis[x]=1;
   bian[x]++;//此牧场被遍历的次数
   for(int i=0;i<G[x].size();i++){
       if(!vis[G[x][i]]){
           dfs(G[x][i]);
       }
   }
}
int main(){
   cin>>k>>n>>m;
   for(int i=1;i<=k;i++){
       cin>>a[i];//每头奶牛所在牧场
   }
   for(int i=1;i<=m;i++){
       int x,y;
       cin>>x>>y;
       G[x].push_back(y);//建边
   }
   for(int i=1;i<=k;i++){
       memset(vis,0,sizeof(vis)); //记住每次遍历前都要先清0
       dfs(a[i]);
   }
   for(int i=1;i<=n;i++){
       if(bian[i]==k) ans++;//key!
   }
   cout<<ans<<endl;
   return 0;
}