本蒟蒻代码不知为何厌氧辣~QAQ 求神犇解惑：

#include<iostream>
#include<cstdio>
#include<stack>
#include<cstring>
#include<vector>
#define N 200001
using namespace std;
template<class T>
inline void ini(T& tmp){
	char buffer;
	tmp=0;
	while(buffer<'0'||buffer>'9')buffer=getchar();
	while(buffer>='0'&&buffer<='9')tmp=tmp*10+buffer-'0',buffer=getchar();
	return;
}
template<class T>
inline void out(T tmp){
	if(tmp>9)out(tmp/10);
	putchar(tmp%10+'0');
}
class Graph{
	public:
		int head[N],w[N],t[N],nxt[N],tot=0;
		void add(int fr,int to,int we){
			tot++;
			nxt[tot]=head[fr];
			w[tot]=we;
			t[tot]=to;
			head[fr]=tot;
		}
}G1;
stack<int>st;
int indexes[N],low[N],cindex=0,cnt=0;
bool instack[N];
vector<int>k[N];
int incomponent[N];
void Tarjan(int i){
	int j;
	low[i]=indexes[i]=cindex++;
	instack[i]=true;
	st.push(i);
	for(int e=G1.head[i];e!=0;e=G1.nxt[e]){
		j=G1.t[e];
		if(indexes[j]==-1){
			Tarjan(j);
			low[i]=min(low[i],low[j]);
		}
		else if(instack[j])low[i]=min(indexes[j],low[i]);
	}
	if(low[i]==indexes[i]){
		cnt++;
		do{
			j=st.top();
			st.pop();
			instack[j]=false;
			k[cnt].push_back(j);
			incomponent[j]=cnt;
		}while(i!=j);
	}
	return;
}
int de[N];
int main(){
	memset(indexes,-1,sizeof(indexes));
	memset(low,-1,sizeof(low));
	memset(de,0,sizeof(de));
	int n,m,a,b;
	scanf("%d%d",&n,&m);
	for(int i=0;i<m;i++){
		ini(a);ini(b);
		G1.add(a,b,0);
	}
	for(int i=1;i<=n;i++)
		if(indexes[i]==-1)
			Tarjan(i);
//	for(int i=1;i<=cnt;i++){
//		for(int j=0;j<k[i].size();j++)printf("%d ",k[i][j]);
//		printf("\n");
//	}
	for(int i=1;i<=n;i++){
		for(int e=G1.head[i];e!=0;e=G1.nxt[e]){
			int j=G1.t[e];
//			G2.add(incomponent[i],incomponent[j],0);
			if(incomponent[j]!=incomponent[i])
			de[incomponent[i]]++;
		}
	}
	int c=0,d=0;
	for(int i=1;i<=cnt;i++){
		if(de[i]==0)
			c+=k[i].size(),d++;
	}
	if(d==1)
	out(c);
	else putchar('0');
}