Problem:
Cycle Detection for a Directed Graph

Find a cycle in a directed graph G(V, E).

Input

A directed graph G is given in the following format:

|V| |E|
s0 t0
s1 t1
:
s|E|-1 t|E|-1

|V| is the number of nodes and |E| is the number of edges in the graph. The graph nodes are named with the numbers 0, 1,..., |V|-1 respectively.

si and ti represent source and target nodes of i-th edge (directed).

Output

Print 1 if G has cycle(s), 0 otherwise.

Constraints

1 ≤ |V| ≤ 100

0 ≤ |E| ≤ 1,000

si ≠ ti

Sample Input 1

3 3
0 1
0 2
1 2

Sample Output 1

0

Sample Input 2

3 3
0 1
1 2
2 0

Sample Output 2

1

Solution in C# (judge verdict: Accepted)

using System;
using System.IO;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using System.Collections.Generic;
using System.Diagnostics;
using Enu = System.Linq.Enumerable;

public class Program
{
    public void Solve()
    {
        int N = Reader.Int(), M = Reader.Int();
        var E = Enu.Range(0, N).Select(i => new List<int>()).ToArray();
        for (int i = 0; i < M; i++)
            E[Reader.Int()].Add(Reader.Int());
        var sorted = TopologicalSort(E);
        Console.WriteLine(sorted == null ? 1 : 0);
        Console.ReadLine();
    }


    int[] TopologicalSort(List<int>[] E)
    {
        var sorted = new List<int>();
        var mark = new byte[E.Length];
        var stack = new Stack<int>();

        for (int v = E.Length - 1; v >= 0; v--)
            if (mark[v] == 0)
            {
                stack.Push(v);
                while (stack.Count > 0)
                {
                    int a = stack.Pop();
                    if (a < 0) { sorted.Add(~a); mark[~a] = 2; continue; }
                    if (mark[a] == 1) return null; // has cycle
                    if (mark[a] == 2) continue;
                    mark[a] = 1;
                    stack.Push(~a);
                    for (int i = E[a].Count - 1; i >= 0; i--)
                        stack.Push(E[a][i]);
                }
            }

        sorted.Reverse();
        return sorted.ToArray();
    }

}


class Entry { static void Main() { new Program().Solve(); } }
class Reader
{
    static TextReader reader = Console.In;
    static readonly char[] separator = { ' ' };
    static readonly StringSplitOptions op = StringSplitOptions.RemoveEmptyEntries;
    static string[] A = new string[0];
    static int i;
    static void Init() { A = new string[0]; }
    public static void Set(TextReader r) { reader = r; Init(); }
    public static void Set(string file) { reader = new StreamReader(file); Init(); }
    public static bool HasNext() { return CheckNext(); }
    public static string String() { return Next(); }
    public static int Int() { return int.Parse(Next()); }
    public static long Long() { return long.Parse(Next()); }
    public static double Double() { return double.Parse(Next()); }
    public static int[] IntLine() { return Array.ConvertAll(Split(Line()), int.Parse); }
    public static int[] IntArray(int N) { return Range(N, Int); }
    public static int[][] IntTable(int H) { return Range(H, IntLine); }
    public static string[] StringArray(int N) { return Range(N, Next); }
    public static string[][] StringTable(int N) { return Range(N, () => Split(Line())); }
    public static string Line() { return reader.ReadLine().Trim(); }
    static string[] Split(string s) { return s.Split(separator, op); }
    static T[] Range<T>(int N, Func<T> f) { var r = new T[N]; for (int i = 0; i < N; r[i++] = f()) ; return r; }
    static string Next() { CheckNext(); return A[i++]; }
    static bool CheckNext()
    {
        if (i < A.Length) return true;
        string line = reader.ReadLine();
        if (line == null) return false;
        if (line == "") return CheckNext();
        A = Split(line);
        i = 0;
        return true;
    }
}