//package whatever; // don't place package name!

import java.io.*;
import java.util.*;

class TopologicalSort {

    static class GraphNode {
        int value;

        List<GraphNode> neighbors;

        public GraphNode(int x) {
            this.value = x;
            this.neighbors = new ArrayList<>();
        }
    }

    static void topologicalSort(GraphNode node) {

        Deque<GraphNode> stack = new ArrayDeque<>();
        Set<Integer> visitedNodes = new HashSet<>();
        topologicalSort(node, stack, visitedNodes);
        while (!stack.isEmpty()) {
            System.out.println(stack.pop().value);
        }

    }

    // 0 : 1, 3
    // 3 : 4
    // 4
    // stack : 2, 1, 4, 3, 0
    static void topologicalSort(GraphNode node, Deque<GraphNode> stack, Set<Integer> visitedNodes) {
        // first mark it as visited
        if (visitedNodes.contains(node.value)) {
            return;
        }

        visitedNodes.add(node.value);
        for (GraphNode neighbor : node.neighbors) {
            topologicalSort(neighbor, stack, visitedNodes);
        }
        stack.offerFirst(node);
    }

    public static void main(String[] args) {
        System.out.println("Hello Java");

        GraphNode zero = new GraphNode(0);
        GraphNode one = new GraphNode(1);
        GraphNode two = new GraphNode(2);
        GraphNode three = new GraphNode(3);
        GraphNode four = new GraphNode(4);

        zero.neighbors.add(three);
        zero.neighbors.add(one);
        one.neighbors.add(two);
        three.neighbors.add(four);
        four.neighbors.add(two);
        topologicalSort(zero);
    }
}