为什么前两个点WA了。。。求大神解释

#include <iostream>
#define N 200005 
#define min(A,B) (A>B?B:A)
using namespace std;
long u[N*2],v[N*2],head[N],_next[N*2],dfn[N],low[N];
bool vis[N],flag[N];
long tot,x,y,n,m,count;
void add(int x,int y)
{
    u[tot]=x;
    v[tot]=y;
    _next[tot]=head[u[tot]];
    head[u[tot]]=tot;
    tot++;
}
void dfs(int cur,int father)
{
    static int index=0;
    int child=0;
    dfn[cur]=low[cur]=++index;
    vis[cur]=1;
    long k=head[cur];
    while(k!=-1)
    {
        if(!vis[v[k]])
        {
            child++;
            dfs(v[k],cur);
            low[cur]=min(low[cur],low[v[k]]);
            if(low[v[k]]>=dfn[cur])
                flag[cur]=1;
        }
        else
            if(dfn[cur]>dfn[v[k]]&&v[k]!=father)
                low[cur]=min(low[cur],dfn[v[k]]);
        k=_next[k];
    }
    if(father<0&&child==1)
        flag[cur]=0;
}
int main()
{
    cin>>n>>m;
    for(int i = 1;i <= n;++i)
        head[i]=-1;
    for(int i = 0;i < m;++i)
    {
        cin>>x>>y;
        add(x,y);
        add(y,x);
    }
    for(int i = 1;i <= n;++i)
        if(!vis[i])
            dfs(i,-1);
    for(int i = 1;i <= n;++i)
        if(flag[i])
            count++;
    cout<<count<<endl;
    for(int i = 1;i <= n;++i)
        if(flag[i])
            cout<<i<<' '; 
    return 0;
}