We're given a hashmap with a key courseId and value a list of courseIds, which represents that the prerequsite of courseId is courseIds. Return a sorted ordering of courses such that we can finish all courses.
Return null if there is no such ordering.
For example, given {'CSC300': ['CSC100', 'CSC200'], 'CSC200': ['CSC100'], 'CSC100': []}, should return ['CSC100', 'CSC200', 'CSCS300'].