Trace numCourses = 4, prerequisites = [[1,0],[2,0],[3,1],[3,2]].
In-degrees: 0:0, 1:1, 2:1, 3:2.
Queue: [0]. Result: [].
Process 0: add to result. Result: [0]. Update in-degrees: 1:0, 2:0. Queue: [1, 2].
Process 1: add to result. Result: [0, 1]. Update: 3:1. Queue: [2].
Process 2: add to result. Result: [0, 1, 2]. Update: 3:0. Queue: [3].
Process 3: add to result. Result: [0, 1, 2, 3].
Length = numCourses. Return [0, 1, 2, 3].
time and space.