91分 第10个点WA 求救qwq

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<stack> 
const int MAXN = 500005;
inline void read(int &x)
{
    char ch = getchar(),c = ch;x = 0;
    while(ch < '0' || ch > '9') c = ch,ch = getchar();
    while(ch <= '9' && ch >= '0') x = (x<<1)+(x<<3)+ch-'0',ch = getchar();
    if(c == '-') x = -x;
}
int head[MAXN],dfn[MAXN],low[MAXN];
int belong[MAXN],sum[MAXN],out[MAXN];
int cnt,dfn_cnt,part,n,m,f,t,ans,flag;
inline int Min(int a,int b)
{return a<b?a:b;}
struct Edge
{
    int f,t,nxt;
}e[MAXN];
inline void insert(int f,int t)
{
    e[++cnt].f = f,e[cnt].t = t;
    e[cnt].nxt = head[f],head[f] = cnt;
}
std::stack <int> s;
void Tarjan(int u)
{
    s.push(u);
    dfn[u] = low[u] = ++ dfn_cnt;
    for(int i = head[u];i != -1;i = e[i].nxt)
    {
        int v = e[i].t;
        if(!dfn[v]){
            Tarjan(v);
            low[v] = Min(low[u],low[v]);
        }
        else if(!belong[v])
            low[u] = Min(low[u],dfn[v]);
    }
    if(dfn[u] == low[u])
    {
        ++ part;
        while(!s.empty())
        {
            int x = s.top();
            s.pop();
            belong[x] = part;
            sum[part] ++;
            if(x == u) break;
        }
    }
}
int main()
{
    memset(head,-1,sizeof(head));
    read(n),read(m);
    for(int i = 1;i <= m;++ i)
    {
        read(f),read(t);
        insert(f,t);
    }
    for(int i = 1;i <= n;++ i)
        if(!dfn[i])
            Tarjan(i);
    for(int i = 1;i <= m;++ i)
        if(belong[e[i].f] != belong[e[i].t])
            out[belong[e[i].f]] ++;
    for(int i = 1;i <= part;++ i)
    {
        if(!flag && !out[i]){
            flag = 1;
            ans = sum[i];
        }
        else if(flag && !out[i]){
            ans = 0;
            break;
        }
    }
    printf("%d\n",ans);
    return 0;
}
看了好几遍感觉并没啥错...并不知道问题在哪QAQ又不给数据下载