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;
using System.Collections.Generic;
using System.Diagnostics;
using System.Globalization;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading;

internal partial class Solver {
    public void Run() {
        var n = ni();
        var m = ni();
        var adj = Enumerable.Range(0, n).Select(_ => new List<int>()).ToArray();
        for (var i = 0; i < m; i++) {
            var a = ni();
            var b = ni();
            adj[a].Add(b);
        }
        var scc = new StronglyConnectedComponentTarjan(adj);
        var g = scc.Run();
        cout.WriteLine(g.Count);
        foreach (var group in g) {
            cout.Write(group.Count);
            foreach (var v in group) {
                cout.Write(" " + v);
            }
            cout.WriteLine();
        }
    }
}

public class StronglyConnectedComponentTarjan {
    private readonly int _n;
    private readonly bool[] _onStack;
    private readonly List<int>[] _adjacent;
    private readonly int[] _indexes, _low;
    private readonly Stack<int> _stack = new Stack<int>();
    private int _index = 0;

    public StronglyConnectedComponentTarjan(List<int>[] adj) {
        _adjacent = adj;
        _n = adj.Length;
        _onStack = new bool[_n];
        _indexes = new int[_n];
        _low = new int[_n];
        _stack = new Stack<int>();
    }

    private void Dfs(int v, List<List<int>> scc) {
        _low[v] = _indexes[v] = ++_index;
        _stack.Push(v);
        _onStack[v] = true;
        foreach (var to in _adjacent[v]) {
            if (_indexes[to] == 0) {
                Dfs(to, scc);
                _low[v] = Math.Min(_low[v], _low[to]);
            } else if (_onStack[to]) {
                _low[v] = Math.Min(_low[v], _indexes[to]);
            }
        }

        if (_low[v] == _indexes[v]) {
            var component = new List<int>();
            while (true) {
                var w = _stack.Pop();
                _onStack[w] = false;
                component.Add(w);
                if (v == w) break;
            }
            scc.Add(component);
        }
    }

    public List<List<int>> Run() {
        _index = 0;
        var scc = new List<List<int>>();
        for (int i = 0; i < _adjacent.Length; i++) {
            if (_indexes[i] == 0) {
                Dfs(i, scc);
            }
        }
        scc.Reverse();
        return scc;
    }

    public int[] GetGroupArray() {
        var component = Run();
        var group = new int[_n];
        for (var i = 0; i < component.Count; i++) {
            foreach (var x in component[i]) {
                group[x] = i;
            }
        }
        return group; // group index is topological order
    }
}
public class StronglyConnectedComponent {
    private readonly int _n;
    private readonly bool[] _isVisited;
    private readonly List<int>[] _adjacent;
    private readonly List<int>[] _inverseAdjacent;
    private readonly Stack<int> _stack = new Stack<int>();

    public StronglyConnectedComponent(List<int>[] adj) {
        _adjacent = adj;
        _n = adj.Length;
        _isVisited = new bool[_n];
        _inverseAdjacent = new List<int>[_n];
        for (var i = 0; i < _n; i++) {
            _inverseAdjacent[i] = new List<int>();
        }

        for (var i = 0; i < _n; i++) {
            foreach (var x in adj[i]) {
                _inverseAdjacent[x].Add(i);
            }
        }
    }

    private void Traverse(int k, List<int>[] adjacent, List<int> label) {
        if (_isVisited[k]) {
            return;
        }
        _stack.Push(k);
        while (_stack.Count > 0) {
            int top = _stack.Pop();
            if (top < 0) {
                label.Add(-top - 1);
                continue;
            }
            if (_isVisited[top]) {
                continue;
            }
            _isVisited[top] = true;
            _stack.Push(-(top + 1));
            foreach (int x in adjacent[top]) {
                if (!_isVisited[x]) {
                    _stack.Push(x);
                }
            }
        }
    }

    public List<List<int>> Run() {
        var label = new List<int>();
        for (var i = 0; i < _n; i++) {
            Traverse(i, _adjacent, label);
        }

        label.Reverse();
        Array.Clear(_isVisited, 0, _isVisited.Length);
        var components = new List<List<int>>();
        foreach (var x in label) {
            if (!_isVisited[x]) {
                var group = new List<int>();
                Traverse(x, _inverseAdjacent, group);
                components.Add(group);
            }
        }
        return components;
    }

    public int[] GetGroupArray() {
        var component = Run();
        var group = new int[_n];
        for (var i = 0; i < component.Count; i++) {
            foreach (var x in component[i]) {
                group[x] = i;
            }
        }
        return group; // group index is topological order
    }
}


// PREWRITEN CODE BEGINS FROM HERE

static public class StringExtensions {
    static public string JoinToString<T>(this IEnumerable<T> source, string separator = " ") {
        return string.Join(separator, source);
    }
}

internal partial class Solver : Scanner {
    static readonly int? StackSizeInMebiByte = null; //50;
    public static void StartAndJoin(Action action, int maxStackSize) {
        var thread = new Thread(new ThreadStart(action), maxStackSize);
        thread.Start();
        thread.Join();
    }

    public static void Main() {
#if LOCAL
        byte[] inputBuffer = new byte[1000000];
        var inputStream = Console.OpenStandardInput(inputBuffer.Length);
        using (var reader = new StreamReader(inputStream, Console.InputEncoding, false, inputBuffer.Length)) {
            Console.SetIn(reader);
            new Solver(Console.In, Console.Out).Run();
        }
#else
        Console.SetOut(new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false });
        if (StackSizeInMebiByte.HasValue) {
            StartAndJoin(() => new Solver(Console.In, Console.Out).Run(), StackSizeInMebiByte.Value * 1024 * 1024);
        } else {
            new Solver(Console.In, Console.Out).Run();
        }
        Console.Out.Flush();
#endif
    }

#pragma warning disable IDE0052
    private readonly TextReader cin;
    private readonly TextWriter cout;
    private readonly TextWriter cerr;
#pragma warning restore IDE0052

    public Solver(TextReader reader, TextWriter writer)
        : base(reader) {
        cin = reader;
        cout = writer;
        cerr = Console.Error;
    }

    public Solver(string input, TextWriter writer)
        : this(new StringReader(input), writer) {
    }

#pragma warning disable IDE1006
#pragma warning disable IDE0051
    private int ni() { return NextInt(); }
    private int[] ni(int n) { return NextIntArray(n); }
    private long nl() { return NextLong(); }
    private long[] nl(int n) { return NextLongArray(n); }
    private double nd() { return NextDouble(); }
    private double[] nd(int n) { return NextDoubleArray(n); }
    private string ns() { return Next(); }
    private string[] ns(int n) { return NextArray(n); }
#pragma warning restore IDE1006
#pragma warning restore IDE0051
}

#if DEBUG
internal static class LinqPadExtension {
    public static string TextDump<T>(this T obj) {
        if (obj is IEnumerable) return (obj as IEnumerable).Cast<object>().JoinToString().Dump();
        else return obj.ToString().Dump();
    }
    public static T Dump<T>(this T obj) {
        return LINQPad.Extensions.Dump(obj);
    }
}
#endif

public class Scanner {
    private readonly TextReader Reader;
    private readonly CultureInfo ci = CultureInfo.InvariantCulture;

    private readonly char[] buffer = new char[2 * 1024];
    private int cursor = 0, length = 0;
    private string Token;
    private readonly StringBuilder sb = new StringBuilder(1024);

    public Scanner()
        : this(Console.In) {
    }

    public Scanner(TextReader reader) {
        Reader = reader;
    }

    public int NextInt() { return checked((int)NextLong()); }
    public long NextLong() {
        var s = Next();
        long r = 0;
        int i = 0;
        bool negative = false;
        if (s[i] == '-') {
            negative = true;
            i++;
        }
        for (; i < s.Length; i++) {
            r = r * 10 + (s[i] - '0');
#if DEBUG
            if (!char.IsDigit(s[i])) throw new FormatException();
#endif
        }
        return negative ? -r : r;
    }
    public double NextDouble() { return double.Parse(Next(), ci); }
    public string[] NextArray(int size) {
        string[] array = new string[size];
        for (int i = 0; i < size; i++) {
            array[i] = Next();
        }

        return array;
    }
    public int[] NextIntArray(int size) {
        int[] array = new int[size];
        for (int i = 0; i < size; i++) {
            array[i] = NextInt();
        }

        return array;
    }

    public long[] NextLongArray(int size) {
        long[] array = new long[size];
        for (int i = 0; i < size; i++) {
            array[i] = NextLong();
        }

        return array;
    }

    public double[] NextDoubleArray(int size) {
        double[] array = new double[size];
        for (int i = 0; i < size; i++) {
            array[i] = NextDouble();
        }

        return array;
    }

    public string Next() {
        if (Token == null) {
            if (!StockToken()) {
                throw new Exception();
            }
        }
        var token = Token;
        Token = null;
        return token;
    }

    public bool HasNext() {
        if (Token != null) {
            return true;
        }

        return StockToken();
    }

    private bool StockToken() {
        while (true) {
            sb.Clear();
            while (true) {
                if (cursor >= length) {
                    cursor = 0;
                    if ((length = Reader.Read(buffer, 0, buffer.Length)) <= 0) {
                        break;
                    }
                }
                var c = buffer[cursor++];
                if (33 <= c && c <= 126) {
                    sb.Append(c);
                } else {
                    if (sb.Length > 0) break;
                }
            }

            if (sb.Length > 0) {
                Token = sb.ToString();
                return true;
            }

            return false;
        }
    }
}