POJ 3180 The Cow Prom

传送门：http://poj.org/problem?id=3180

解题思路：

#include <cstdio>
#include <algorithm>
#include <cstring>
#include <map>
#include <iostream>
using namespace std;

const int MAXN=10010;
const int MAXM=50010;

struct Edge{
    int to;
    int next;
}edge[MAXM];

int head[MAXN];
int tot;

void addEdge(int u,int v){
    edge[tot].to=v;
    edge[tot].next=head[u];
    head[u]=tot++;
}

void init(){
    memset(head,-1,sizeof(head));
    tot=0;
}

int LOW[MAXN],DFS[MAXN],Stack[MAXN],Num[MAXN],Belong[MAXN];
bool Instack[MAXN];
int Index;
int scc;
int top;

void Targan(int u){
    LOW[u]=DFS[u]=++Index;
    Stack[top++]=u;
    Instack[u]=true;

    int v;
    for(int i=head[u];i!=-1;i=edge[i].next){
        v=edge[i].to;
        if(!DFS[v]){
            Targan(v);
            if(LOW[u]>LOW[v])
                LOW[u]=LOW[v];
        }else if(Instack[v]&&LOW[u]>DFS[v]){
                LOW[u]=DFS[v];
        }
    }

    if(LOW[u]==DFS[u]){
        scc++;

        do{
            v=Stack[--top];
            Instack[v]=false;
            Belong[v]=scc;
            Num[scc]++;
        }while(v!=u);
    }
}

void Solve(int n){
    memset(Instack,false,sizeof(Instack));
    memset(DFS,0,sizeof(DFS));
    memset(Num,0,sizeof(Num));

    Index=scc=top=0;
    for(int i=1;i<=n;i++)
        if(!DFS[i])
        Targan(i);
}

int main(){
    int n,m;
    scanf("%d%d",&n,&m);
    init();
    for(int i=0;i<m;i++){
        int u,v;
        scanf("%d%d",&u,&v);
        addEdge(u,v);
    }
    Solve(n);
    int ans=0;
    for(int i=1;i<=scc;i++)
        if(Num[i]>=2)
        ans++;
    cout<<ans<<endl;
}