Problem:
MathJax.Hub.Config({ tex2jax: { inlineMath: [["$","$"], ["\\(","\\)"]], processEscapes: true }});

Topological Sort

A directed acyclic graph (DAG) can be used to represent the ordering of tasks. Tasks are represented by vertices and constraints where one task can begin before another, are represented by edges. For example, in the above example, you can undertake task B after both task A and task B are finished. You can obtain the proper sequence of all the tasks by a topological sort.

Given a DAG $G$, print the order of vertices after the topological sort.

Input

A directed graph $G$ is given in the following format:

$|V|\;|E|$

$s_0 \; t_0$

$s_1 \; t_1$

:

$s_{|E|-1} \; t_{|E|-1}$

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

$s_i$ and $t_i$ represent source and target nodes of $i$-th edge (directed).

Output

Print the vertices numbers in order. Print a number in a line.

If there are multiple possible solutions, print any one of them (the solution is judged by a special validator).

Constraints

$1 \leq |V| \leq 10,000$

$0 \leq |E| \leq 100,000$

There are no parallel edges in $G$

There are no self loops in $G$

Sample Input

6 6
0 1
1 2
3 1
3 4
4 5
5 2

Sample Output

0
3
1
4
5
2

Solution in JAVA (judge verdict: Accepted)

import java.util.ArrayList;
import java.util.LinkedList;
import java.util.Scanner;
import java.util.Stack;

public class Main {
	
	int totalVertex;
	ArrayList<LinkedList<Integer>> adjList;
	
	public void loadAdjList() {
		Scanner in = new Scanner(System.in);
		totalVertex = in.nextInt();
		int totalEdges = in.nextInt();
		adjList = new ArrayList<>();
		
		for(int i = 0; i < totalVertex; i++) {
			adjList.add(new LinkedList<Integer>());
		}
		for(int i = 0; i < totalEdges; i++) {
			int source = in.nextInt();
			int edge = in.nextInt();
			adjList.get(source).add(edge);
		}
		in.close();
	}
	
	public void topoSort() {
		Stack<Integer> stack = new Stack<>();
		boolean[] visited = new boolean[totalVertex];
		for(int i = 0; i < totalVertex; i++) {
			if(!visited[i]) { //runs DFS on all nodes (even unconnected graphs)
				DFS(i, visited, stack);
			}
		}
		while(!stack.isEmpty()) {
			System.out.println(stack.pop());
		}
	}
	public void DFS(int node, boolean[] visited, Stack<Integer> stack) {
		visited[node] = true;
		LinkedList<Integer> neighbors = adjList.get(node);
		for(Integer n : neighbors){ //alwasy use enhanced for loop for LL bc otherwise running n squared
			if(!visited[n]) {
				DFS(n, visited, stack); //this callse dfs on all unvisited neighbors
			}
		}
		stack.push(node); //now we are allowed to put the original node on the stack bc we finished traversing the childresn
	}
	
	public static void main(String[] args) {
		Main Graph = new Main();
		Graph.loadAdjList();
		Graph.topoSort();
	}

}