Can every job finish? (cycle detection)

You have n jobs labeled 0 .. n-1 and a list of prerequisite pairs. [a, b] means job b must finish before job a starts. Decide whether all jobs can be completed, that is, whether the dependencies are free of cycles.

n = 4, prereqs = [[1, 0], [2, 0], [3, 1], [3, 2]]   ->   True
n = 2, prereqs = [[1, 0], [0, 1]]                    ->   False   (0 and 1 wait on each other)

Return True if a valid schedule exists, else False. Aim for $O(V + E)$.