Course Schedule II

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**.