210.课程表Ⅱ

 public int[] findOrder(int numCourses, int[][] prerequisites) {
        int[] ans = new int[numCourses];
        for (int i = 0; i < numCourses; i++) {
            ans[i] = i;
        }
        if (prerequisites == null || prerequisites.length == 0) {
            return ans;
        }
        HashMap<Integer, Node> nodes = new HashMap<>();
        // 先把图建好
        for (int[] prerequisite : prerequisites) {
            int to = prerequisite[0];
            int from = prerequisite[1];
            if (!nodes.containsKey(to)) {
                nodes.put(to, new Node(to));
            }
            if (!nodes.containsKey(from)) {
                nodes.put(from, new Node(from));
            }
            Node t = nodes.get(to);
            Node f = nodes.get(from);
            f.nexts.add(t);
            t.in++;
        }
        int needNum = nodes.size();
        Queue<Node> zeroInQueue = new LinkedList<>();
        int index = 0;
        for (int i = 0; i < numCourses; i++) {
            if (!nodes.containsKey(i)) {
                ans[index++] = i;
            } else {
                if (nodes.get(i).in == 0) {
                    zeroInQueue.add(nodes.get(i));
                }
            }
        }
        int count = 0;
        while (!zeroInQueue.isEmpty()) {
            Node cur = zeroInQueue.poll();
            ans[index++] = cur.name;
            count++;
            for (Node next : cur.nexts) {
                if (--next.in == 0) {
                    zeroInQueue.add(next);
                }
            }
        }
        return count == needNum ? ans : new int[0];
    }
    // 一个node，就是一个课程
    // name是课程的编号
    // in是课程的入度
    public static class Node {
        public int name;
        public int in;
        public ArrayList<Node> nexts;
        public Node(int n) {
            name = n;
            in = 0;
            nexts = new ArrayList<>();
        }
    }