题解 | #拓扑排序#

import java.util.HashSet;
import java.util.Scanner;
import java.util.Set;

class Vertex {
    public int inDegree = 0;
    public Set<Vertex> nexts = new HashSet<>();
    public boolean isRemaining = true;
}
public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        final int N = in.nextInt();
        final int M = in.nextInt();

        Vertex[] vertexs = new Vertex[N];
        for (int i = 0; i != N; ++i) {
            vertexs[i] = new Vertex();
        }

        for (int i = 0; i != M; ++i) {
            int start = in.nextInt() - 1;
            int end = in.nextInt() - 1;

            vertexs[start].nexts.add(vertexs[end]);
            vertexs[end].inDegree++;
        }

        int numOfRemaining = N;
        int[] result = new int[N];
        int k = 0;
        while (numOfRemaining > 0) {
            boolean found = false;

            for (int i = 0; i != N; ++i) {
                Vertex v = vertexs[i];
                if (v.isRemaining && v.inDegree == 0) {
                    found = true;
                    v.isRemaining = false;
                    for (Vertex next : v.nexts) {
                        next.inDegree--;
                    }
                    numOfRemaining--;
                    result[k++] = i + 1;
                }
            }

            if (!found) {
                break;
            }
        }

        if (numOfRemaining == 0) {
            StringBuilder sb = new StringBuilder();
            for (int x : result) {
                sb.append(x + " ");
            }
            System.out.println(sb.toString().trim());
        } else {
            System.out.println("-1");
        }
    }
}