Kosaraju's Algorithm for SCCs

#include <iostream>
#include <algorithm>
#include <vector>
#include <map>
#include <set>
#include <array>
#include <stack>
#include <queue>
#include <random>
#include <numeric>
#include <functional>
#include <chrono>
#include <utility>
#include <iomanip>
#include <assert.h>

using namespace std;

void dbg_out() { cerr << endl; }
template<typename Head, typename... Tail>
void dbg_out(Head H, Tail... T) { cerr << ' ' << H; dbg_out(T...); }
#define dbg(...) cerr << "(" << #__VA_ARGS__ << "):", dbg_out(__VA_ARGS__)

#define rng_init mt19937 rng(chrono::steady_clock::now().time_since_epoch().count())
#define rng_seed(x) mt19937 rng(x)
#define all(x) (x).begin(), (x).end()
#define sz(x) (int) (x).size()
// #define int long long

const int MXN = 5e5 + 5, INF = 1e9 + 5;
vector<int> g[MXN], gr[MXN];
vector<int> topo_order;
int vis[MXN];
vector<vector<int>> components;

void dfs1(int u) {
    vis[u] = 1;

    for (const auto &v : g[u]) {
        if (vis[v]) continue;
        dfs1(v);
    }

    topo_order.push_back(u);
}

void dfs2(int u) {
    vis[u] = true;

    components.back().push_back(u);

    for (const auto &v : gr[u]) {
        if (vis[v]) continue;
        dfs2(v);
    }
}

void solve() {
    int N, M;
    cin >> N >> M;

    while (M--) {
        int u, v;
        cin >> u >> v;

        g[u].push_back(v);
        gr[v].push_back(u);
    }

    for (int i = 0; i < N; i++) {
        if (!vis[i]) dfs1(i);
    }

    reverse(all(topo_order));

    for (int i = 0; i < N; i++)
        vis[i] = false;

    for (const auto &node : topo_order) {
        if (vis[node]) continue;

        components.emplace_back();
        dfs2(node);
    }

    cout << sz(components) << "\n";

    for (const auto &component : components) {
        cout << sz(component) << " ";

        for (const auto &v : component)
            cout << v << " ";

        cout << "\n";
    }
}

signed main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);

    int TC = 1;
    // cin >> TC;
    while (TC--) solve();
}