Course Schedule II

Statement

Let’s assume that there are a total of $n$ courses labeled from $0$ to $n-1$. Some courses may have prerequisites. A list of prerequisites is specified such that if $Prerequisites_i = {a, b}$, you must take course $b$ before course $a$.

Given the total number of courses $n$ and a list of the prerequisite pairs, return the course order a student should take to finish all of the courses. If there are multiple valid orderings of courses, then the return any one of them.

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

Constraints:

Let $n$ be the number of courses.

• $1 \leq n \leq 1500$
• $0 \leq$ prerequisites.length $\leq 1000$
• prerequisites[i].length $== 2$
• $0 \leq a, \space b < n$
• $a \neq b$
• All the pairs $[a, \space b]$ are distinct.

Examples