tarjan求强连通分量

//#include<bits/stdc++.h>
#include<iostream>
#include<cstdio>
#include<iomanip>
#include<cmath>
#include<algorithm>
#include<vector>
#include<cstring>
#include<queue>
#include<map>
#include<string>
#include<stack>
#define fi first
#define se second
#define INF 0x3f3f3f3f
#define fio ios::sync_with_stdio(false);cin.tie(0);cout.tie(0)
#define pqueue priority_queue
#define NEW(a,b) memset(a,b,sizeof(a))
#define lowbit(x) ((x)&(-x))
const double pi=4.0*atan(1.0);
const int maxn=1e5+8;
typedef long long LL;
typedef unsigned long long ULL;
const LL mod=1e9+7;
const ULL base=1e7+7;
using namespace std;
struct node{
    int to,nxt;
}g[maxn*2];
int head[maxn];
int cnt=0;
void add(int u,int v){
    g[cnt].to=v;
    g[cnt].nxt=head[u];
    head[u]=cnt++;
}
int deep=0;
int dfn[maxn],low[maxn];
bool vis[maxn];
int Stack[maxn];
int tail=0;
int ans=0;
void tarjan(int u){
    dfn[u]=++deep;
    low[u]=deep;
    Stack[++tail]=u;
    vis[u]=1;
    int t=head[u];
    int r=0;
    while(t!=-1){
        if(!dfn[g[t].to]){
            tarjan(g[t].to);
            low[u]=min(low[u],low[g[t].to]);
        }
        else{
            if(vis[g[t].to]){
                low[u]=min(low[u],low[g[t].to]);
            }
        }
        t=g[t].nxt;
    }
    if(dfn[u]==low[u]){
        while(Stack[tail]!=u){
            vis[Stack[tail]]=0;
            tail--;
            r=1;
        }
        vis[Stack[tail]]=0;
        tail--;
    }
    ans+=r;
}
int main(){
    memset(head,-1,sizeof(head));
    int n,m,x,y;
    scanf("%d%d",&n,&m);
    for(int i=1;i<=m;i++){
        scanf("%d%d",&x,&y);
        add(x,y);
    }
    for(int i=1;i<=n;i++){
        if(!dfn[i]){
            tarjan(i);
        }
    }
    cout<<ans<<endl;
}