Return a valid course completion order

You are given an integer `n` representing `n` courses labeled `0..n-1` and a list of prerequisite pairs `prerequisites`, where each pair `[a, b]` means you must complete course `b` before course `a`. Return any valid ordering of courses you can take to finish all courses. If it is impossible due to cyclic prerequisites, return an empty list. Requirements: - The solution should run efficiently for large `n` and prerequisite lists.