input.txt

5 4
3 1
3 2
4 3
5 3

Main.java

import java.util.*;

public class Main {
  static Map<Integer, List<Integer>> map;
  static Map<Integer, Integer> dp;

  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    int m = sc.nextInt();

    map = new HashMap<>();
    dp  = new HashMap<>();

    for (int i = 1; i <= n; i++)
      map.put(i, new ArrayList<Integer>());

    for (int i = 0; i < m; i++) {
      int a = sc.nextInt();
      int b = sc.nextInt();
      map.get(b).add(a);
    }

    List<Pair> list = new ArrayList<>();

    for (int i = 1; i <= n; i++)
      list.add(new Pair(i, numberOfHacking(i)));

    int max = list.stream()
                  .max((p1, p2) -> Integer.compare(p1.b, p2.b))
                  .get()
                  .b;

    list.stream()
        .filter(p -> p.b == max)
        .forEach(p -> System.out.print(p.a + " "));

    System.out.println("\b");
  }

  static int numberOfHacking(int n) {
    if (dp.get(n) != null)
      return dp.get(n);

    List<Integer> list = map.get(n);
    if (list.size() == 0)
      return 1;
    else {
      dp.put(n, 1 + list.stream().mapToInt(x -> numberOfHacking(x)).sum());
      return dp.get(n);
    }
  }
}

class Pair {
  int a;
  int b;

  Pair(int a, int b) {
    this.a = a;
    this.b = b;
  }

  public String toString() {
    return "(" + a + ", " + b + ")";
  }
}

Makefile

run: compile
	@java Main < input.txt

compile:
	@javac Main.java