bzoj 1051: [HAOI2006]受欢迎的牛 tarjan缩点

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cstring>
#include<queue>
#include<map>
#include<set>
using namespace std;
#define MAXN 11000
#define MAXV MAXN
#define MAXE MAXN*20
struct Edge
{
        int np;
        Edge *next;
}E[MAXE],*V[MAXV];
int tope=-1;
void addedge(int x,int y)
{
        E[++tope].np=y;
        E[tope].next=V[x];
        V[x]=&E[tope];
}
int low[MAXN],dfn[MAXN];
int dfstime=0;
int stack[MAXN];
int tops=-1;
int color[MAXN],topc=0;
int size_c[MAXN];
int size_sub[MAXN];
bool vis[MAXN];
set<int> S[MAXN];
void tarjan(int now)
{
        low[now]=dfn[now]=++dfstime;
        Edge *ne;
        stack[++tops]=now;
        for (ne=V[now];ne;ne=ne->next)
        {
                if (vis[ne->np])continue;
                if (dfn[ne->np])
                {
                        low[now]=min(low[now],dfn[ne->np]);
                }else
                {
                        tarjan(ne->np);
                        low[now]=min(low[now],low[ne->np]);
                }
        }
        if (low[now]==dfn[now])
        {
                ++topc;
                while (stack[tops]!=now)
                {
                        vis[stack[tops]]=true;
                        color[stack[tops--]]=topc;
                        size_c[topc]++;
                }
                vis[stack[tops]]=true;
                color[stack[tops--]]=topc;
                size_c[topc]++;
        }
}
pair<int,int> edge[MAXE];
int topedge;
int degree[MAXN];
queue<int> Q;
int main()
{
        freopen("input.txt","r",stdin);
        int n,m;
        scanf("%d%d",&n,&m);
        int i,j,k,x,y,z;
        for (i=0;i<m;i++)
        {
                scanf("%d%d",&x,&y);
                addedge(x,y);
                edge[i].first=x;
                edge[i].second=y;
        }
        sort(edge,&edge[m]);
        topedge=0;
        for (i=1;i<m;i++)
        {
                if (edge[i]!=edge[topedge])
                        edge[++topedge]=edge[i];
        }
        for (i=1;i<=n;i++)
        {
                if (!dfn[i])tarjan(i);
        }
    //    memset(V,0,sizeof(V));
    //    tope=-1;
        for (i=0;i<m;i++)
        {
                if (color[edge[i].first]==color[edge[i].second])continue;
        //        if (S[color[edge[i].first]].find(color[edge[i].second])!=S[color[edge[i].first]].end())continue;
    //            addedge(color[edge[i].first],color[edge[i].second]);
        //        S[color[edge[i].first]].insert(color[edge[i].second]);
                degree[color[edge[i].first]]++;
        }
        int ans;
        ans=0;
        for (i=1;i<=topc;i++)
        {
                if (degree[i]==0)ans++,x=i;
        }
        if (ans==1)
        {
                printf("%d\n",size_c[x]);
        }else
        {
                printf("0\n");
        }
        return 0;
}