You are given n courses, labeled from 0 to n - 1. Some courses have prerequisites, which are provided as a list of pairs: prerequisites[i] $= [a, b]$. To take course $a$, you must first complete course $b$.

Your task is to determine a valid order in which you can complete all the courses and return it as an array of course labels.

• If there are multiple valid orderings, you can return any of them.

• If it’s impossible to finish all courses (due to a cycle in prerequisites), return an empty array.

Note: There can be a course in the $0$ to $n−1$ range with no prerequisites.