BZOJ1051|HAOI2006受欢迎的牛|强连通分量

#include<iostream>
#include<cstdio>
#include<cstdlib>
using namespace std;

int n,m;
int tot=0,cnt=0,top=0;
struct node{
       int to,next;
}e[50001],d[50001];

int head[10001];
int dfn[10001],low[10001],q[10001];
int scc=0,h[10001],belong[10001],hav[10001];//scc是强连通分量的个数 
bool vis[10001],inq[10001];
int ans=0;

void dfs(int a)
{
     int now;
     inq[a]=vis[a]=1;
     low[a]=dfn[a]=++cnt;
     q[++top]=a;
     int c=head[a];
     while(c)
     {
        if (!vis[e[c].to])
        {
            dfs(e[c].to);
            low[a]=min(low[a],low[e[c].to]);
        }
        else if (inq[e[c].to]) low[a]=min(low[a],dfn[e[c].to]);
        c=e[c].next;
    }
    if (low[a]==dfn[a])
    {
       scc++;
       while (now!=a)
       {
             now=q[top--];
             inq[now]=0;
             belong[now]=scc;
             ++hav[scc];
       }
    }
}             

void rebuild()
{
     cnt=0;
     for (int i=1; i<=n; i++)
     {
         int c=head[i];
         while (c)
         {
               if (belong[e[c].to]!=belong[i])
               {
                  d[++cnt].to=belong[e[c].to];
                  d[cnt].next=h[i];
                  h[i]=cnt;
               }
               c=d[c].next;
         }
     }
}

void work()
{
    for (int i=1; i<=scc; i++)
        if (!h[i])
                 {
                             if (ans)
                             {
                                       ans=0; return;
                          }
                          else ans=hav[i];
                          if (i==1) cout << hav[i]<< endl; 
                 }
      
}

void tarjan()
{
       for (int i=1; i<=n; i++) if (!vis[i]) dfs(i);
    rebuild();
}

int main()
{
    
    cin >> n >> m;
    for (int i=1; i<=m; i++) 
    {
        int x,y,z;
        cin >> x >> y;
        e[++tot].to=y; e[tot].next=head[x]; head[x]=tot;
    }
    tarjan();
    work();
    cout << ans;
    system("pause");
    return 0;
}