[poj] 2618 popular cows

#include<cstdio>
#include<algorithm>
#include<stack>
#define N 10010
#define M 50010
using namespace std;
int n,m,head[N],dfn[N],low[N],cnt=1,t,sum,bel[N],num[N],out[N],ans;
bool instk[N];
stack <int> stk;
struct hhh
{
    int to,next;
}edge[M];

int read()
{
    int ans=0,fu=1;
    char j=getchar();
    for (;(j<‘0‘ || j>‘9‘) && j!=‘-‘;j=getchar()) ;
    if (j==‘-‘) fu=-1,j=getchar();
    for (;j>=‘0‘ && j<=‘9‘;j=getchar()) ans*=10,ans+=j-‘0‘;
    return ans*fu;
}

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

void Tarjan(int x)
{
    dfn[x]=low[x]=++t;
    stk.push(x);
    instk[x]=1;
    int v;
    for (int i=head[x];i;i=edge[i].next)
    {
    v=edge[i].to;
    if (!dfn[v])
    {
        Tarjan(v);
        low[x]=min(low[x],low[v]);
    }
    else if (instk[v]) low[x]=min(low[x],dfn[v]);
    }
    if (dfn[x]==low[x])
    {
    sum++;
    do
    {
        v=stk.top();
        stk.pop();
        instk[v]=0;
        bel[v]=sum;
        num[sum]++;
    }while(v!=x);
    }
}

int main()
{
    n=read();
    m=read();
    for (int i=1,a,b;i<=m;i++)
    {
    a=read();
    b=read();
    add(a,b);
    }
    for (int i=1;i<=n;i++)
    if (!dfn[i]) Tarjan(i);
    for (int i=1;i<=n;i++)
    for (int j=head[i],v;j;j=edge[j].next)
    {
        v=edge[j].to;
        if (bel[i]!=bel[v]) out[bel[i]]++;
    }
    for (int i=1;i<=sum;i++)
    {
    if (!out[i] && ans)
    {
        ans=0;
        break;
    }
    else if (!out[i]) ans=num[i];
    }
    printf("%d",ans);
    return 0;
}