Problem - Course Schedule

You have N courses ($0$ to $N-1$) and a list of prerequisites. For example, [$0$, $1$] means you must take course $1$ before course $0$.

Return true if you can finish all courses, false if impossible.

Think: what makes it impossible to finish all courses? If course A requires B and B requires A, you have a circular dependency. No valid ordering exists. This is asking you to detect whether a directed graph contains a cycle. The graph has courses as nodes and prerequisites as directed edges.