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 Rust (1.42.0) (judge verdict: Accepted)

use proconio::*;

pub struct Csr<E> {
    start: Vec<usize>,
    elist: Vec<E>,
}

impl<E> Csr<E>
where
    E: Copy,
{
    pub fn new(n: usize, edges: &[(usize, E)], init: E) -> Self {
        let mut csr = Csr {
            start: vec![0; n + 1],
            elist: vec![init; edges.len()],
        };
        for e in edges.iter() {
            csr.start[e.0 + 1] += 1;
        }
        for i in 1..=n {
            csr.start[i] += csr.start[i - 1];
        }
        let mut counter = csr.start.clone();
        for e in edges.iter() {
            csr.elist[counter[e.0]] = e.1;
            counter[e.0] += 1;
        }
        csr
    }
}

#[derive(Copy, Clone)]
struct _Edge {
    to: usize,
}

/// Reference:
/// R. Tarjan,
/// Depth-First Search and Linear Graph Algorithms
pub struct SccGraph {
    n: usize,
    edges: Vec<(usize, _Edge)>,
}

impl SccGraph {
    pub fn new(n: usize) -> Self {
        SccGraph { n, edges: vec![] }
    }

    pub fn num_vertices(&self) -> usize {
        self.n
    }

    pub fn add_edge(&mut self, from: usize, to: usize) {
        self.edges.push((from, _Edge { to }));
    }

    /// return pair of (# of scc, scc id)
    pub fn scc_ids(&self) -> (usize, Vec<usize>) {
        // In C++ ac-library, this function is implemented by using recursive lambda functions.
        // Instead, we use fn and struct for capturing environments.
        struct _Env {
            g: Csr<_Edge>,
            now_ord: usize,
            group_num: usize,
            visited: Vec<usize>,
            low: Vec<usize>,
            ord: Vec<Option<usize>>,
            ids: Vec<usize>,
        }
        let mut env = _Env {
            g: Csr::new(self.n, &self.edges, _Edge { to: 0 }),
            now_ord: 0,
            group_num: 0,
            visited: Vec::with_capacity(self.n),
            low: vec![0; self.n],
            ord: vec![None; self.n],
            ids: vec![0; self.n],
        };

        fn dfs(v: usize, n: usize, env: &mut _Env) {
            env.low[v] = env.now_ord;
            env.ord[v] = Some(env.now_ord);
            env.now_ord += 1;
            env.visited.push(v);

            for i in env.g.start[v]..env.g.start[v + 1] {
                let to = env.g.elist[i].to;
                if let Some(x) = env.ord[to] {
                    env.low[v] = std::cmp::min(env.low[v], x);
                } else {
                    dfs(to, n, env);
                    env.low[v] = std::cmp::min(env.low[v], env.low[to]);
                }
            }
            if env.low[v] == env.ord[v].unwrap() {
                loop {
                    let u = *env.visited.last().unwrap();
                    env.visited.pop();
                    env.ord[u] = Some(n);
                    env.ids[u] = env.group_num;
                    if u == v {
                        break;
                    }
                }
                env.group_num += 1;
            }
        }
        for i in 0..self.n {
            if env.ord[i].is_none() {
                dfs(i, self.n, &mut env);
            }
        }
        for x in env.ids.iter_mut() {
            *x = env.group_num - 1 - *x;
        }
        (env.group_num, env.ids)
    }

    pub fn scc(&self) -> Vec<Vec<usize>> {
        let ids = self.scc_ids();
        let group_num = ids.0;
        let mut counts = vec![0usize; group_num];
        for &x in ids.1.iter() {
            counts[x] += 1;
        }
        let mut groups: Vec<Vec<usize>> = (0..ids.0).map(|_| vec![]).collect();
        for i in 0..group_num {
            groups[i].reserve(counts[i]);
        }
        for i in 0..self.n {
            groups[ids.1[i]].push(i);
        }
        groups
    }
}

fn main() {
    input! {
    n: usize,
    m: usize,
    e: [(usize, usize); m],
    }
    let mut scc = SccGraph::new(n);
    for (s, t) in e {
        scc.add_edge(s, t);
    }

    let groups = scc.scc();

    println!("{}", groups.len());
    for i in 0..groups.len() {
        print!("{}", groups[i].len());
        for j in &groups[i] {
            print!(" {}", j);
        }
        println!();
    }
}