Course Schedule II - The Constraint

Each prerequisite pair $[a, b]$ means "course $b$ must be taken before course $a$". This creates a directed edge $b \to a$ in your graph. If you have $[0, 1]$ and $[1, 2]$, the graph looks like $2 \to 1 \to 0$. You must take course $2$ first, then $1$, then $0$. The order respects all dependencies.

What if you have $[0, 1]$ and $[1, 0]$? Course $0$ needs $1$ first, and course $1$ needs $0$ first. That is $0 \to 1 \to 0$, a cycle. No valid ordering exists, so you return an empty array. Building the adjacency list and in-degree array from the input pairs is the first step before running Kahn's algorithm.