poj 2186 Popular Cows【tarjan求scc个数&&缩点】【求一个图中可以到达其余所有任意点的点的个数】

#include<stdio.h>
#include<string.h>
#include<algorithm>
#include<vector>
#include<stack>
#define MAX 50010
#define INF 0x3f3f3f
using namespace std;
struct node
{
    int beg,end,next;
}edge[MAX];
int low[MAX],dfn[MAX];
int n,m,ans;
int sccno[MAX],instack[MAX];
int dfsclock,scccnt;
vector<int>newmap[MAX];
vector<int>scc[MAX];
int head[MAX];
int out[MAX];
stack<int>s;
int sum,sumout,ant;
void init()
{
    ans=0;
    memset(head,-1,sizeof(head));
}
void add(int u,int v)
{
    edge[ans].beg=u;
    edge[ans].end=v;
    edge[ans].next=head[u];
    head[u]=ans++;
}
void getmap()
{
    int a,b,i;
    while(m--)
    {
        scanf("%d%d",&a,&b);
        add(a,b);
    }
}
void tarjan(int u)
{
    int v,i,j;
    s.push(u);
    instack[u]=1;
    dfn[u]=low[u]=++dfsclock;
    for(i=head[u];i!=-1;i=edge[i].next)
    {
        v=edge[i].end;
        if(!dfn[v])
        {
            tarjan(v);
            low[u]=min(low[u],low[v]);
        }
        else if(instack[v])
            low[u]=min(low[u],dfn[v]);
    }
    if(dfn[u]==low[u])
    {
        scccnt++;
        while(1)
        {
            v=s.top();
            s.pop();
            instack[v]=0;
            sccno[v]=scccnt;
            if(v==u)
            break;
        }
    }
}
void find(int l,int r)
{
    memset(low,0,sizeof(low));
    memset(dfn,0,sizeof(dfn));
    memset(instack,0,sizeof(instack));
    memset(sccno,0,sizeof(sccno));
    dfsclock=scccnt=0;
    for(int i=l;i<=r;i++)
    {
        if(!dfn[i])
            tarjan(i);
    }
}
void suodian()
{
    int i;
    ant=0;
    for(i=1;i<=scccnt;i++)
    {
        newmap[i].clear();
        out[i]=0;
    }
    for(i=0;i<ans;i++)
    {
        int u=sccno[edge[i].beg];
        int v=sccno[edge[i].end];
        if(u!=v)
        {
            newmap[u].push_back(v);
            out[u]++;
        }
    }
}
void solve()
{
    int i,j;
	sumout=0;sum=0;
	for(i=1;i<=scccnt;i++)
	{
		if(!out[i])
		{
			sumout++;
			ant=i;
		}
	}
	if(sumout==0)
	{
		printf("%d\n",n);
		return ;
	}
    else if(sumout==1)
    {
    	for(i=1;i<=n;i++)
    	    if(sccno[i]==ant)
    	        sum++;
    	printf("%d\n",sum); 
    }
	else 
	printf("0\n");
}
int main()
{
    int t;
    while(scanf("%d%d",&n,&m)!=EOF)
    {
        init();
        getmap();
        find(1,n);
        suodian();
        solve();
    }
    return 0;
}