tarjan+topsort

题目

---

缩完点后统计入读为零的点就可以来。

因为缩完点后肯定是DAG

#include<iostream>
#include<cstdio>
#include<algorithm>
using namespace std;
int dfn[100100],low[100010],tim;
bool instack[100010];
int stack[100010],top,num;
int belong[100010] ;
struct node
{
	int point;
	int next;
};
node line[100010];
int head[100010],tail;
void add(int x,int y)
{
	line[++tail].point=y;
	line[tail].next=head[x];
	head[x]=tail;
	return ;
}
void tarjan(int x)
{
	dfn[x]=low[x]=++tim;
	stack[++top]=x;
	instack[x]=true;
	int need=head[x];
	while(need)
	{
		if(!dfn[line[need].point])
		{
			tarjan(line[need].point);
			if(low[line[need].point]<low[x])
				low[x]=low[line[need].point];
		}
		else
		if(instack[line[need].point]&&dfn[line[need].point]<low[x])
			low[x]=dfn[line[need].point];
		need=line[need].next;
	}
	if(low[x]==dfn[x])
	{
		num+=1;
		do
		{
			need=stack[top--];
			instack[need]=false;
			belong[need]=num;
		}while(x!=need);
	}
}
int ru[100010];
int main()
{
	int n;
	scanf("%d",&n);
	int a;
	for(int i=1;i<=n;i++)
	{
		scanf("%d",&a);
		while(a!=0)
		{
			add(i,a);
			scanf("%d",&a);
		}
	}
	int ans=0;
	for(int i=1;i<=n;i++)
		if(!dfn[i])
			tarjan(i);
	for(int i=1;i<=n;i++)
	{
		int need=head[i];
		while(need)
		{
			if(belong[i]!=belong[line[need].point])
				ru[belong[line[need].point]]+=1;
			need=line[need].next;
		}
	}
	for(int i=1;i<=num;i++)
		if(!ru[i])
			ans+=1;
	printf("%d",ans);
}