求助！代码未知问题！

#include<bits/stdc++.h>
using namespace std;
const int N = 5e4+5; 
int n,m;
struct Edge{
	int to,next;
}edge[N*4];
int cn(0),head[N];
void add(int u,int v){
	edge[++cn].to = v;
	edge[cn].next = head[u];
	head[u] = cn;
}
int dfn[N],low[N],dfs(0),sta[N],top(0);
int co[N],col(0),poc[N]; 
void tarjan(int u){
	dfn[u] = low[u] = ++dfs;
	sta[++top] = u;
	for (int i=head[u];i;i=edge[i].next){
		int v=edge[i].to;
		if (!dfn[i]){
			tarjan(v);
			low[u] = min(low[u],low[v]);
		}
		else if (!co[v])
			low[u] = min(low[u],dfn[v]);
	}
	if (low[u]==dfn[u]){
		co[u] = ++col;
		++poc[col];
		while(sta[top]!=u){
			++poc[col];
			co[sta[top]] = col;
			--top;
		}
		--top;
	}
}
int d[N];
int main(){
	ios_base::sync_with_stdio(false);
	cin.tie(0); cout.tie(0);
	cin >> n >> m;
	for (int i=1,u,v;i<=m;i++){
		cin >> u >> v;
		add(v,u);
	}
	for (int i=1;i<=n;i++)
		if (!dfn[i])
			tarjan(i);
	for (int i=1;i<=n;i++)
		for (int j=head[i];j;j=edge[i].next)
			if (co[i] != co[edge[j].to])
				d[co[edge[j].to]]++;
	int ans(0),u(0);
	for (int i=1;i<=col;i++)
		if (!d[i]) ans=poc[i],u++;
	if (u==1) cout << ans;
	else cout << 0;
	return 0;
}