Find a valid course order from prerequisites

You are given `numCourses` labeled from `0` to `numCourses-1` and a list of prerequisite pairs `prerequisites`, where each pair `[a, b]` means you must complete course `b` before you can take course `a`. ## Task Return **one valid order** in which you can take all courses. - If there are multiple valid answers, return any one of them. - If it is **impossible** to finish all courses (because prerequisites contain a cycle), return an **empty list**. ## Input - `numCourses`: integer - `prerequisites`: list of pairs `[a, b]` ## Output - A list of length `numCourses` representing a valid topological ordering, or `[]` if none exists. ## Constraints (reasonable interview defaults) - `1 <= numCourses <= 2 * 10^5` - `0 <= len(prerequisites) <= 2 * 10^5` - `0 <= a, b < numCourses` ## Example Input: `numCourses = 4`, `prerequisites = [[1,0],[2,0],[3,1],[3,2]]` Output: `[0,1,2,3]` (or `[0,2,1,3]`)