TopologicalSort.java

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

public class Topo {
    private static final int WHITE = 0;
    private static final int GRAY = 1;
    private static final int BLACK = 2;

    private static void dfs(int v, List<List<Integer>> edges, int[] state, List<Integer> sorting) {
        state[v] = GRAY;
        List<Integer> children = edges.get(v);
        for (Integer child : children) {
            if (state[child] == GRAY)
                throw new RuntimeException(String.format("Loop from %d to %d", v, child));
            if (state[child] == BLACK)
                continue;
            if (state[child] == WHITE)
                dfs(child, edges, state, sorting);
        }
        state[v] = BLACK;
        sorting.add(v);
    }

    private static List<Integer> topoSort(List<List<Integer>> edges, int N) {
        List<Integer> sorting = new ArrayList<>(N);
        int[] states = new int[N]; // all are white
        for (int i = 0; i < N; i++) {
            if (states[i] == WHITE)
                dfs(i, edges, states, sorting);
        }
        return sorting;
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        int e = scanner.nextInt();
        List<List<Integer>> edges = new ArrayList<>();
        for (int i = 0; i < n; i++) {
            edges.add(new ArrayList<>());
        }
        for (int i = 0; i < e; i++) {
            int from = scanner.nextInt() - 1;
            int to = scanner.nextInt() - 1;
            edges.get(from).add(to);
        }

        List<Integer> results = topoSort(edges, n);

        for (int i = results.size() - 1; i >= 0; i--) {
            System.out.format("%d ", results.get(i) + 1);
        }
    }
}

/*

n e

e lines with `from` `to`
5 5

1 2
1 3
2 3
4 3
4 5

 */