Practice - Course Schedule II

Try this practice problem: you have $n$ courses and $m$ prerequisites. Each prerequisite $[a, b]$ means you must take course $b$ before course $a$. Return any valid ordering of courses, or an empty array if it is impossible. This is topological sort. Build a directed graph where $b \to a$ for each prerequisite.

Run Kahn's or DFS-based topological sort. If the graph has a cycle, return empty. Otherwise, return the topological order. This problem tests your ability to recognize DAG structure and apply topological sort.