图与遍历：查路线、找连接、排任务顺序

import java.util.ArrayList;
import java.util.List;
import java.util.PriorityQueue;
public class Main {
    static List<Integer> order(int n, int[][] edges) {
        if (n < 0 || edges == null) throw new IllegalArgumentException("input");
        List<List<Integer>> graph = new ArrayList<>();
        for (int i = 0; i < n; i++) graph.add(new ArrayList<>());
        int[] indegree = new int[n];
        for (int[] edge : edges) {
            if (edge == null || edge.length != 2 || edge[0] < 0 || edge[0] >= n
                    || edge[1] < 0 || edge[1] >= n) throw new IllegalArgumentException("edge");
            graph.get(edge[0]).add(edge[1]);
            indegree[edge[1]] = Math.addExact(indegree[edge[1]],1);
        }
        PriorityQueue<Integer> ready = new PriorityQueue<>();
        for (int i = 0; i < n; i++) if (indegree[i] == 0) ready.add(i);
        List<Integer> result = new ArrayList<>();
        while (!ready.isEmpty()) {
            int current = ready.remove();
            result.add(current);
            // 每条边只移除一次；恰好变成零才将后继加入候选。
            for (int next : graph.get(current))
                if (--indegree[next] == 0) ready.add(next);
        }
        if (result.size() != n) throw new IllegalArgumentException("cycle");
        return List.copyOf(result);
    }
    static void check(List<Integer> wanted, int n, int[][] edges) {
        if (!wanted.equals(order(n,edges))) throw new AssertionError(wanted);
    }
    static void reject(int n, int[][] edges) {
        try { order(n,edges); } catch (IllegalArgumentException e) { return; }
        throw new AssertionError("must reject");
    }
    public static void main(String[] args) {
        // 共同前置、独立点、空图和重复边都固定顺序。
        check(List.of(0,1,2,3),4,new int[][]{{0,2},{1,2},{2,3}});
        check(List.of(0,1,2),3,new int[0][]);
        check(List.of(),0,new int[0][]);
        check(List.of(0,1),2,new int[][]{{0,1},{0,1}});
        // 普通环、自环和越界端点分别拒绝。
        reject(2,new int[][]{{0,1},{1,0}});
        reject(1,new int[][]{{0,0}});
        reject(2,new int[][]{{0,2}});
        System.out.println("PASS 7");
    }
}