有关数组大小的问题

RT，这份代码将 N 设置为 3*10^6 时会WA后两个点，设置为6*10^6时可AC，但本题中边数只有5*10^5,这是为什么呢

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<vector>
#define ll long long
#define ull unsigned long long
#define gc getchar
using namespace std;
int read(){
    int x=0,f=1;
    char c=getchar();
    while(c<'0'||c>'9'){if(c=='-')f=-1;c=getchar();}
    while(c>='0'&&c<='9') x=(x<<1)+(x<<3)+(c^48),c=getchar();
    return x*f;
}
const int N = 3000007;
int cnt[N],root,n,m,tot,head[N],nxt[N],to[N],dfn[N],low[N],vis[N];
void add(int x,int y){
    to[++tot] = y , nxt[tot] = head[x] , head[x] = tot;
}
int cur = 0;
void dfs(int x,int in_edge){
    dfn[x] = low[x] = ++cur;
    for(int i=head[x];i;i=nxt[i]){
        int y = to[i];
        //if(i == (in_edge ^ 1)) continue;
        //if(dfn[y]) low[x] = min(low[x],dfn[y]); //
        if(!dfn[y]){
            dfs(y,i);
            low[x] = min(low[x],low[y]);
            if(dfn[x] < low[y]){
                vis[i] = vis[i^1] = 1;
            }

        } 
        else if(i != (in_edge ^ 1)){
            low[x] = min(low[x],dfn[y]);
        }
    }
}
int count_dcc = 0;
vector<int> a[500007];
void sfd(int x){
    cnt[x] = 1;
    a[count_dcc].push_back(x);
    for(int i=head[x];i;i=nxt[i]){
        int y = to[i];
        if(cnt[y] || vis[i]) continue;
        sfd(y); 
    }
} 
int main(){
    int n = read() , m =read();
    tot = 1;
    for(int i=1;i<=m;i++){
        int u = read() , v = read();
        add(u,v),add(v,u); 
    }
    for(int i=1;i<=n;i++){
        if(!dfn[i]) dfs(i,0);
    }

    for(int i=1;i<=n;i++){
        if(!cnt[i]){
            count_dcc++;
            sfd(i);     
        }
    }
    cout << count_dcc<<endl;
    for(int i=1;i<=count_dcc;i++){
        cout<<a[i].size()<<" ";
        for(int j=0;j<a[i].size();j++){
            cout<<a[i][j]<<" ";
        }
        cout<<endl;
    }
    return 0;
}