# 0210. Course Schedule II

<https://leetcode.com/problems/course-schedule-ii>

## Description

There are a total of `numCourses` courses you have to take, labeled from `0` to `numCourses - 1`. You are given an array `prerequisites` where `prerequisites[i] = [ai, bi]` indicates that you **must** take course `bi` first if you want to take course `ai`.

* For example, the pair `[0, 1]`, indicates that to take course `0` you have to first take course `1`.

Return *the ordering of courses you should take to finish all courses*. If there are many valid answers, return **any** of them. If it is impossible to finish all courses, return **an empty array**.

**Example 1:**

```
**Input:** numCourses = 2, prerequisites = [[1,0]]
**Output:** [0,1]
**Explanation:** There are a total of 2 courses to take. To take course 1 you should have finished course 0. So the correct course order is [0,1].
```

**Example 2:**

```
**Input:** numCourses = 4, prerequisites = [[1,0],[2,0],[3,1],[3,2]]
**Output:** [0,2,1,3]
**Explanation:** There are a total of 4 courses to take. To take course 3 you should have finished both courses 1 and 2. Both courses 1 and 2 should be taken after you finished course 0.
So one correct course order is [0,1,2,3]. Another correct ordering is [0,2,1,3].
```

**Example 3:**

```
**Input:** numCourses = 1, prerequisites = []
**Output:** [0]
```

**Constraints:**

* `1 <= numCourses <= 2000`
* `0 <= prerequisites.length <= numCourses * (numCourses - 1)`
* `prerequisites[i].length == 2`
* `0 <= ai, bi < numCourses`
* `ai != bi`
* All the pairs `[ai, bi]` are **distinct**.

## ac1: Topological sorting + BFS

if return empty array `[]` : **return new int\[0]**

```java
class Solution {
    public int[] findOrder(int numCourses, int[][] prerequisites) {
        int[] res =  new int[numCourses]; int index = 0;
        // have graph
        List<List<Integer>> adjList = new ArrayList<List<Integer>>();
        for (int i = 0; i < numCourses; i++) adjList.add(new ArrayList<Integer>());

        // indegree
        int[] indegree = new int[numCourses];
        for (int[] pre : prerequisites) {
            indegree[pre[0]]++;
            adjList.get(pre[1]).add(pre[0]);
        }

        // BFS
        Queue<Integer> q = new LinkedList<Integer>();
        for (int i = 0; i < indegree.length; i++) {
            if (indegree[i] == 0) {
                q.offer(i);
            }
        }
        while (!q.isEmpty()) {
            int head = q.poll();
            res[index++] = head;
            for (int next : adjList.get(head)) {
                indegree[next]--;
                if (indegree[next] == 0) {
                    q.offer(next);
                }
            }
        }

        // result
        return index == numCourses ? res : new int[0];
    }
}
```


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