class Solution:
    def findOrder(self, numCourses: int, prerequisites: List[List[int]]) -> List[int]:
        return self.bfs(numCourses, prerequisites)


    def bfs(self, num_courses, prerequisites):
        next_courses, indegree = self.get_indegree(num_courses, prerequisites)
        start_courses = [course for course, degree in indegree.items() if degree == 0]
        learned_courses = [] 

        queue = collections.deque(start_courses)
        visited = set()

        while queue:
            course = queue.popleft()
            learned_courses.append(course)
            visited.add(course)
            for next_course in next_courses[course]:
                indegree[next_course] -= 1
                if indegree[next_course] == 0 and next_course not in visited:
                    queue.append(next_course)

        else:
            return learned_courses if num_courses == len(learned_courses) else []



    def get_indegree(self, num_courses, prerequisites):
        next_courses = {x:[] for x in range(num_courses)}
        indegree = {x:0 for x in range(num_courses)}

        for course, next_course in prerequisites:
            next_courses[next_course].append(course)
            indegree[course] += 1

        # print(next_courses, indegree)
        return next_courses, indegree