若连通块个数为num，为什么不能统计入度为num-1的个数呢（实测68分55）

#include<bits/stdc++.h>
#define maxn 100005
using namespace std;
int cnt,num,n,m,du[maxn],h[maxn],h1[maxn],tot,tot1,t,dfn[maxn],low[maxn],be[maxn];
stack<int>q;
bool f[maxn];
struct qwq
{int next,to;
}edge[1000005],edge1[1000005];
struct qaq
{int x,y;
bool operator < (qaq b)const{
        return x<b.x||(x==b.x&&y<b.y);
    }
    bool operator==(const qaq&b)const{
    return x==b.x&&y==b.y;
    }
}edge2[1000005];
inline bool cmp(qaq a,qaq b){return a.x<b.x||(a.x==b.x&&a.y<b.y);}
inline void add(int x,int y)
{edge[++tot].next=h[x];
 h[x]=tot;
 edge[tot].to=y;
}
inline void dfs(int k)
{dfn[k]=low[k]=++cnt;f[k]=1;q.push(k);
 for(int i=h[k];i;i=edge[i].next)
 {int p=edge[i].to;
  if(!dfn[p]){dfs(p);low[k]=min(low[k],low[p]);
  }
  else if(f[p]){low[k]=min(low[k],dfn[p]);
  }
 }
 if(dfn[k]==low[k])
 {++num;int s=0;
  while(s!=k)
  {s=q.top();q.pop();f[s]=0;
   be[s]=num;
  } 	
 }
}
int main()
{scanf("%d%d",&n,&m);
 for(int i=1;i<=m;i++)
 {int x,y;
  scanf("%d%d",&x,&y);
  if(x!=y) add(x,y);
 }
 for(int i=1;i<=n;i++)if(!dfn[i])dfs(i);
 for(int i=1;i<=n;i++)
  for(int j=h[i];j;j=edge[j].next)
   {int p=edge[j].to;
    if(be[p]==be[i]) continue;
    edge2[++t].x=be[i];
    edge2[t].y=be[p];
   } 
  sort(edge2+1,edge2+t+1);  
  int mm=unique(edge2+1,edge2+t+1)-edge2;
  for(int i=1;i<mm;i++)
  {edge1[++tot1].next=h1[edge2[i].x];
   edge1[tot1].to=edge2[i].y;
   h1[edge2[i].x]=tot1;
   du[edge2[i].y]++;	
  }
  int ans=0;
  for(int i=1;i<=n;i++) if(du[be[i]]==num-1)ans++;
  printf("%d",ans);
}