poj 2186 Popular Cows 强连通

#include<cstdio>
#include<algorithm>
#include<vector>
#include<cstring>
#include<stack>
using namespace std;

const int maxn = 10000 + 10;

vector<int> G[maxn], G2[maxn];
vector<int> S;
int vis[maxn], sccno[maxn], scc_cnt;

int max(int a,int b)
{
	if(a>b) return a;
	else return b;
}
void dfs1(int u) 
{
    if(vis[u]) return;
    vis[u] = 1;
    for(int i = 0; i < G[u].size(); i++) dfs1(G[u][i]);
    S.push_back(u);
}

void dfs2(int u) 
{
    if(sccno[u]) return;
    sccno[u] = scc_cnt;
    for(int i = 0; i < G2[u].size(); i++) dfs2(G2[u][i]);
}

void find_scc(int n) 
{
	int i;
    scc_cnt = 0;
    S.clear();
    memset(sccno, 0, sizeof(sccno));
    memset(vis, 0, sizeof(vis));
    for(i = 0; i < n; i++) dfs1(i);
    for(i = n-1; i >= 0; i--)
    if(!sccno[S[i]]) { scc_cnt++; dfs2(S[i]); }
}
int main()
{
	int n,m;
	while(scanf("%d%d",&n,&m)!=EOF)
	{
		int x,y,i,j,v;
		for(i=1;i<=m;i++)
		{
			scanf("%d%d",&x,&y);
			x--;y--;
			G[x].push_back(y);
			G2[y].push_back(x);
		}
		int out[maxn];
		memset(out,0,sizeof(out));
		find_scc(n); 
		int cnt=0,ans=0,ob;
		for(i=0;i<n;i++)
		{
			for(j=0;j<G[i].size();j++)
			{
				v=G[i][j];
				if(sccno[i]!=sccno[v])
					out[sccno[i]]++;
			}
		}
		for(i=1;i<=scc_cnt;i++)
			if(out[i]==0)
			{
				ob=i;
				cnt++;
			}
		if(cnt>1) printf("0
");
		else
		{
			for(i=0;i<n;i++)
			{
				if(sccno[i]==ob)
				{
					ans++;
//					printf("%d ",i);
				}
			}
			printf("%d
",ans);
		}
	}
	return 0;
}