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)

// https://atcoder.jp/contests/practice2/tasks/practice2_g
//
#![allow(unused_imports)]
use std::io::*;
use std::io::Write;
use std::fmt::*;
use std::str::*;
use std::cmp::*;
use std::collections::*;

// Input macros.
// Original by tanakh: https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8
#[allow(unused_macros)]
macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
    ($($r:tt)*) => {
        let s = {
            use std::io::Read;
            let mut s = String::new();
            std::io::stdin().read_to_string(&mut s).unwrap();
            s
        };
        let mut iter = s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
}

#[allow(unused_macros)]
macro_rules! input_inner {
    ($iter:expr) => {};
    ($iter:expr, ) => {};

    ($iter:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($iter, $t);
        input_inner!{$iter $($r)*}
    };

    ($iter:expr, mut $var:ident : $t:tt $($r:tt)*) => {
        let mut $var = read_value!($iter, $t);
        input_inner!{$iter $($r)*}
    };
}

#[allow(unused_macros)]
macro_rules! read_value {
    ($iter:expr, ( $($t:tt),* )) => {
        ( $(read_value!($iter, $t)),* )
    };

    ($iter:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($iter, $t)).collect::<Vec<_>>()
    };

    ($iter:expr, [ next / $t:tt ]) => {
        {
            let len = read_value!($iter, usize);
            (0..len).map(|_| read_value!($iter, $t)).collect::<Vec<_>>()
        }
    };

    ($iter:expr, switch) => {
        {
            let ty = read_value!($iter, i32);
            if ty == 1 {
                vec![ty, read_value!($iter, i32), read_value!($iter, i32)]
            } else if ty == 2 {
                vec![ty, read_value!($iter, i32)]
            } else {
                vec![ty, read_value!($iter, i32)]
            }
        }
    };

    ($iter:expr, chars) => {
        read_value!($iter, String).chars().collect::<Vec<char>>()
    };

    ($iter:expr, usize1) => {
        read_value!($iter, usize) - 1
    };

    ($iter:expr, $t:ty) => {
        $iter.next().unwrap().parse::<$t>().expect("Parse error")
    };
}

#[allow(unused_macros)]
macro_rules! read_line {
    ($t:tt) => {
        {
            let mut s = String::new();
            std::io::stdin().read_line(&mut s).unwrap();
            s.trim_right().parse::<$t>().unwrap()
        }
    }
}

#[allow(unused_macros)]
macro_rules! dvec {
    ($t:expr ; $len:expr) => {
        vec![$t; $len]
    };

    ($t:expr ; $len:expr, $($rest:expr),*) => {
        vec![dvec!($t; $($rest),*); $len]
    };
}

#[allow(unused_macros)]
macro_rules! ifv {
    ($t:expr, $a:expr, $b: expr) => {
        if $t { $a } else { $b }
    }
}

#[allow(unused_macros)]
macro_rules! fill {
    ($t:expr, $v:expr) => {
        for i in 0..$t.len() {
            $t[i] = $v;
        }
    };
}

#[allow(unused_macros)]
macro_rules! join {
    ($t:expr, $glue:expr) => {
        $t.into_iter().map(|w| w.to_string()).collect::<Vec<_>>().join($glue)
    };
}

#[allow(unused_macros)]
macro_rules! debug {
    ($($a:expr),*) => {
        println!(concat!($(stringify!($a), " = {:?}, "),*), $($a),*);
    }
}


type DirectedGraph = Vec<Vec<usize>>;

struct StronglyConnectedComponent {
    n: usize,
    graph: DirectedGraph,
    reversed_graph: DirectedGraph,
    component_ids: Vec<usize>
}

impl StronglyConnectedComponent {
    fn new(g: DirectedGraph) -> Self {
        let n = g.len();
        let mut rg = vec![vec![]; n];
        for i in 0..n {
            for &to in &g[i] {
                rg[to].push(i);
            }
        }
        Self {
            n: g.len(),
            graph: g,
            reversed_graph: rg,
            component_ids: vec![]
        }
    }

    fn decompose(&mut self) -> usize {
        let mut visited = vec![false; self.n];
        let mut rev_order = vec![];
        for i in 0..self.n {
            if !visited[i] {
                self.dfs(i, &mut visited, &mut rev_order);
            }
        }

        let mut visited = vec![false; self.n];
        let mut id = 0;
        let mut component_ids = vec![0; self.n];
        for &r in rev_order.iter().rev() {
            if !visited[r] {
                self.rdfs(r, id, &mut visited, &mut component_ids);
                id += 1;
            }
        }
        self.component_ids = component_ids;
        id
    }

    fn dfs(&self, now: usize, visited: &mut Vec<bool>, rev_order: &mut Vec<usize>) {
        visited[now] = true;
        for &to in &self.graph[now] {
            if !visited[to] {
                self.dfs(to, visited, rev_order);
            }
        }
        rev_order.push(now);
    }

    fn rdfs(&self, now: usize, id: usize, visited: &mut Vec<bool>, component_ids: &mut Vec<usize>) {
        visited[now] = true;
        component_ids[now] = id;
        for &to in &self.reversed_graph[now] {
            if !visited[to] {
                self.rdfs(to, id, visited, component_ids);
            }
        }
    }
}

//===

// ===

fn main() {
    input! {
        n: usize, m: usize,
        edges: [(usize, usize); m]
    };
    let mut g = vec![vec![]; n];
    for (u, v) in edges {
        if u != v {
            g[u].push(v);
        }
    }
    let mut scc = StronglyConnectedComponent::new(g);
    let ng = scc.decompose();
    // debug!(scc.component_ids);

    let mut cids = vec![vec![]; ng];
    for i in 0..n {
        cids[scc.component_ids[i]].push(i);
    }
    println!("{}", ng);
    for cm in cids {
        println!("{} {}", cm.len(), join!(cm, " "));
    }
}