Problem:
Score : 100 points

Problem Statement

You are given a directed graph with N vertices and M edges, not necessarily simple. The i-th edge is oriented from the vertex a_i to the vertex b_i.
Divide this graph into strongly connected components and print them in their topological order.

Constraints

1 \leq N \leq 500,000

1 \leq M \leq 500,000

0 \leq a_i, b_i < N

Input

Input is given from Standard Input in the following format:

N M
a_0 b_0
a_1 b_1
:
a_{M - 1} b_{M - 1}

Output

Print 1+K lines, where K is the number of strongly connected components.
Print K on the first line.
Print the information of each strongly connected component in next K lines in the following format, where l is the number of vertices in the strongly connected component and v_i is the index of the vertex in it.

l v_0 v_1 ... v_{l-1}

Here, for each edge (a_i, b_i), b_i should not appear in earlier line than a_i.

If there are multiple correct output, print any of them.

Sample Input 1

6 7
1 4
5 2
3 0
5 5
4 1
0 3
4 2

Sample Output 1

4
1 5
2 4 1
1 2
2 3 0

Solution in C++ (GCC 9.2.1) (judge verdict: Accepted)

#include<bits/stdc++.h>
using namespace std;
#define ALL(x) x.begin(),x.end()
#define rep(i,n) for(int i=0;i<(n);i++)
#define debug(v) cout<<#v<<":";for(auto x:v){cout<<x<<' ';}cout<<endl;
#define mod 1000000007
using ll=long long;
const int INF=1000000000;
const ll LINF=1001002003004005006ll;
int dx[]={1,0,-1,0},dy[]={0,1,0,-1};
// ll gcd(ll a,ll b){return b?gcd(b,a%b):a;}
template<class T>bool chmax(T &a,const T &b){if(a<b){a=b;return true;}return false;}
template<class T>bool chmin(T &a,const T &b){if(b<a){a=b;return true;}return false;}
 
struct IOSetup{
    IOSetup(){
        cin.tie(0);
        ios::sync_with_stdio(0);
        cout<<fixed<<setprecision(12);
    }
} iosetup;

template<typename T1,typename T2>
ostream &operator<<(ostream &os,const pair<T1,T2>&p){
    os<<p.first<<" "<<p.second;
    return os;
}
 
template<typename T>
ostream &operator<<(ostream &os,const vector<T>&v){
    for(int i=0;i<(int)v.size();i++) os<<v[i]<<(i+1==(int)v.size()?"":" ");
    return os;
}

template<typename T1,typename T2>
istream &operator>>(istream &is,pair<T1,T2>&p){
    is>>p.first>>p.second;
    return is;
}

template<typename T>
istream &operator>>(istream &is,vector<T>&v){
    for(T &x:v)is>>x;
    return is;
}

struct StronglyConnectedComponents{
    vector<vector<int>> g;
    vector<vector<int>> to,from;
    vector<int> comp,order,used;
    StronglyConnectedComponents(vector<vector<int>> &g):
    g(g),to(g.size()),from(g.size()),comp(g.size(),-1),used(g.size()){
        for(int i=0;i<g.size();i++){
            for(auto x:g[i]){
                to[i].push_back(x);
                from[x].push_back(i);
            }
        }
    }
    int operator[](int k){
        return comp[k];
    }
    void dfs(int now){
        if(used[now]) return ;
        used[now]=true;
        for(int x:to[now])dfs(x);
        order.push_back(now);
    }
    void rdfs(int now,int cnt){
        if(comp[now]!=-1) return ;
        comp[now]=cnt;
        for(int x:from[now]) rdfs(x,cnt);
    }
    vector<vector<int>> build(){
        vector<vector<int>> t;
        for(int i=0;i<to.size();i++) dfs(i);
        reverse(order.begin(),order.end());
        int ptr=0;
        for(int i:order)if(comp[i]==-1)rdfs(i,ptr),ptr++;
 
        t.resize(ptr);
        for(int i=0;i<g.size();i++){
            for(auto v:to[i]){
                int x=comp[i],y=comp[v];
                if(x==y) continue;
                t[x].push_back(y);
            }
        }
        return t;
    }
};

signed main(){
    int n,m;cin>>n>>m;
    vector<vector<int>> g(n);
    rep(i,m){
        int u,v;cin>>u>>v;
        g[u].push_back(v);
    }

    StronglyConnectedComponents scc(g);
    auto t=scc.build();
    vector<vector<int>> belong(t.size());
    rep(i,n)belong[scc[i]].push_back(i);
    cout<<belong.size()<<endl;
    rep(i,belong.size()){
        cout<<belong[i].size();
        for(auto u:belong[i]) cout<<" "<<u;
        cout<<endl;
    }
    return 0;
}