BZOJ1051 [HAOI2006]受欢迎的牛

#include<cstdio> 
#include<cstring>  
int idx,ans2,low[50001],deep[50001],idx2,cnt,last[50001],x[50001],y[50001],z[50001],top,f[50001],ans[50001],num[50001],n,m;  
bool inz[50001]; 
struct edge
{
    int to,next;
}e[50001];  
int min(int a,int b)  
{  
    if(a<b)
        return a;  
    return b;
}  
void addage(int x,int y)  
{  
     e[++idx].to=y; 
     e[idx].next=last[x]; 
     last[x]=idx;  
}   
void tarjan(int x)  
{  
    low[x]=deep[x]=++cnt;  
    z[++top]=x;   
    inz[x]=1;   
    for(int i=last[x];i>=0;i=e[i].next) 
    {  
        if(!deep[e[i].to])  
         {  
            tarjan(e[i].to);  
            low[x]=min(low[x],low[e[i].to]);  
          }  
        else if(inz[e[i].to]&&low[x]>deep[e[i].to]) 
              low[x]=deep[e[i].to];   
    }   
    if(low[x]==deep[x])  
     {  
        idx2++;
        int t=-1;  
        while(x!=t)
        {  
            t=z[top--];  
            ans[idx2]++;  
            f[t]=idx2;  
            inz[t]=0;  
        } 
     }    
}   
int main()  
{   
    scanf("%d%d",&n,&m); 
    memset(last,-1,sizeof(last)); 
    for(int i=1;i<=m;i++)  
    {  
        scanf("%d%d",&x[i],&y[i]);  
        addage(x[i],y[i]);  
    }
    for(int i=1;i<=n;i++)  
         if(!deep[i]) 
             tarjan(i);    
    for(int i=1;i<=m;i++)  
         if(f[x[i]]!=f[y[i]])  
               num[f[x[i]]]++;   
    for(int i=1;i<=idx2;i++)  
      if(!num[i])  
       {  
            if(ans2)  
            {
                printf("0"); 
                return 0;
            }  
            ans2=i;  
       }  
    printf("%d",ans[ans2]);   
}