还是要参加NOIP —— 洛谷 P2341 [HAOI2006]受欢迎的牛

#include <ctype.h>
#include <cstdio>
#define M 50050
#define N 10050
void read(int &x)
{
    x=0;bool f=0;
    char ch=getchar();
    while(!isdigit(ch))
    {
        if(ch==‘-‘) f=1;
        ch=getchar();
    }
    while(isdigit(ch))
    {
        x=x*10+ch-‘0‘;
        ch=getchar(); 
    }
    x=f?(~x)+1:x;
}
struct node
{
    int pre,next,to;
}edge[M];
int min(int a,int b) {return a>b?b:a;} 
int QLT[N],ans,num,out[N],n,m,head[M],cnt,dfn[N],sumcol,col[N],low[N],T,Stack[N<<1],top;
bool vis[N],instack[N];
void add(int i,int j)
{
    cnt++;
    edge[cnt].pre=i;
    edge[cnt].next=head[i];
    edge[cnt].to=j;
    head[i]=cnt;
}
void tarjan(int x)
{
    low[x]=dfn[x]=++T;
    Stack[++top]=x;
    vis[x]=1;
    instack[x]=1;
    for(int i=head[x];i;i=edge[i].next)
    {
        int v=edge[i].to;
        if(instack[v]) low[x]=min(low[x],dfn[v]);
        else if(!vis[v])
        {
            tarjan(v);
            low[x]=min(low[x],low[v]);
        }
    }
    if(dfn[x]==low[x])
    {
        sumcol++;
        while(x!=Stack[top])
        {
            QLT[sumcol]++;
            instack[Stack[top]]=0;
            col[Stack[top--]]=sumcol;
        }
        instack[Stack[top]]=0;
        col[Stack[top--]]=sumcol;
        QLT[sumcol]++;
    }
}
int main()
{
    read(n);read(m);
    for(int x,y,i=1;i<=m;i++)
    {
        read(x);
        read(y);
        add(x,y); 
    }
    for(int i=1;i<=n;i++) if(!vis[i]) tarjan(i);
    for(int i=1;i<=m;i++)
    {
        int u=edge[i].pre,v=edge[i].to;
        if(col[v]!=col[u])
            out[col[u]]++;
    }
    for(int i=1;i<=sumcol;i++) if(!out[i]) num++,ans=QLT[i];
    num!=1?printf("0"):printf("%d\n",ans);
    return 0;
}