Course Schedule — Can You Finish All Courses?

Question

## Problem

There are `n` courses (0 to n-1). You are given `prerequisites[i] = [a, b]` meaning you must take course `b` before `a`. Return `true` if you can finish all courses.

## Example
```
n = 2, prerequisites = [[1,0]]
Output: true

n = 2, prerequisites = [[1,0],[0,1]]
Output: false  -- circular dependency
```

## Constraints
- 1 ≤ n ≤ 2000
- 0 ≤ prerequisites.length ≤ 5000

## Follow-up
Return the actual course order (topological sort). If multiple valid orderings exist, return any one.