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# (.NET Core 3.1.201) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;

class G
{
	static int[] Read() => Console.ReadLine().Split().Select(int.Parse).ToArray();
	static void Main()
	{
		var h = Read();
		var n = h[0];
		var es = new int[h[1]].Select(_ => Read()).ToArray();

		var map = Array.ConvertAll(new int[n], _ => new List<int>());
		var map_r = Array.ConvertAll(new int[n], _ => new List<int>());
		foreach (var e in es)
		{
			map[e[0]].Add(e[1]);
			map_r[e[1]].Add(e[0]);
		}

		var u = new bool[n];
		var order = new List<int>();
		var gs = new List<List<int>>();
		List<int> lg = null;

		Action<int> Dfs = null;
		Dfs = v =>
		{
			u[v] = true;
			foreach (var nv in map[v]) if (!u[nv]) Dfs(nv);
			order.Add(v);
		};
		for (int v = 0; v < n; v++) if (!u[v]) Dfs(v);

		Action<int> Dfs_r = null;
		Dfs_r = v =>
		{
			u[v] = true;
			foreach (var nv in map_r[v]) if (!u[nv]) Dfs_r(nv);
			lg.Add(v);
		};
		Array.Clear(u, 0, n);
		order.Reverse();
		foreach (var v in order) if (!u[v]) { gs.Add(lg = new List<int>()); Dfs_r(v); }

		Console.WriteLine(gs.Count);
		Console.WriteLine(string.Join("\n", gs.Select(g => string.Join(" ", g.Prepend(g.Count)))));
	}
}