[洛谷p2002]消息扩散

题目链接：###

点我

题目分析：###

tarjan缩点后统计入度为0的连通分量。

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代码：###

#include<bits/stdc++.h>
#define MAXN 100000+5
#define MAXM 500000+5
using namespace std;
inline int read(){
	int cnt=0,f=1;char c;
	c=getchar();
	while(!isdigit(c)){
		if(c=='-')f=-f;
		c=getchar();
	}
	while(isdigit(c)){
		cnt=cnt*10+c-'0';
		c=getchar();
	}
	return cnt*f;
}
int n,dfn[MAXN],nxt[MAXM],first[MAXN],to[MAXM],low[MAXN],in[MAXN],id[MAXN];
int sign,sta[MAXN];bool insta[MAXN];
int tot,cnt,top;

void add(int x,int y){
	nxt[++tot]=first[x];
	first[x]=tot;
	to[tot]=y;
}
void dfs(int u){
	dfn[u]=low[u]=++sign;
	sta[++top]=u,insta[u]=1;
	for(register int i=first[u];i;i=nxt[i]){
		int t=to[i];
		if(!dfn[t]) dfs(t),low[u]=min(low[u],low[t]);
		else if(insta[t]&&dfn[t]<low[u])low[u]=dfn[t];
	}
	if(low[u]==dfn[u]){
		cnt++; do{
			id[sta[top]]=cnt,insta[sta[top]]=0;
		}while(sta[top--]!=u);
	}	
}
int ans=0;int m;
int main(){
	n=read();m=read();
	for(register int i=1;i<=m;i++){
	    int x=read();
	    int y=read();
	    add(x,y);
	}
	for(register int i=1;i<=n;i++)if(!dfn[i])dfs(i);
	for(register int i=1;i<=n;i++)
		for(register int j=first[i];j;j=nxt[j]){
			int v=to[j];
			if(id[v]!=id[i])in[id[v]]++;
		}
	for(register int i=1;i<=cnt;i++)if(!in[i])ans++;
	printf("%d",ans);
	return 0;
}