TopologicalSort.java

import java.util.*;

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

    private static void dfs(int v, List<List<Integer>> edges, int[] colors, List<Integer> queue) {
        colors[v] = GRAY;
        List<Integer> list = edges.get(v);
        for (int to : list) {
            if (colors[to] == GRAY)
                throw new RuntimeException(String.format("Cycle from %d to %d!", v, to));

            if (colors[to] == WHITE)
                dfs(to, edges, colors, queue);
        }
        colors[v] = BLACK;
        queue.add(v);
    }

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

    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> result = topoSort(edges, n);
        for (int i = result.size() - 1; i >= 0; i--) {
            System.out.format("%d ", result.get(i) + 1);
        }
    }
}
/*
n e
edges…


5 5

1 2
2 3
1 3
4 3
4 5

 */