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

        prereq_map = collections.defaultdict(list)
        indegree = collections.defaultdict(int)

        for crs, prereq in prerequisites:
            # build the adjancency list
            prereq_map[crs].append(prereq)
            # Record each node's in-degree
            indegree[prereq] += 1

        # Queue for maintainig list of nodes that have 0 in-degree
        q = deque([k for k in range(numCourses) if k not in indegree])
        output = []

        # Until there are nodes in the Q
        while q:
            # Pop one node with 0 in-degree
            node = q.popleft()
            output.append(node)

            # Reduce in-degree for all the neighbors
            for nei in prereq_map[node]:
                indegree[nei] -= 1

                # Add neighbor to Q if in-degree becomes 0
                if indegree[nei] == 0:
                    q.append(nei)

        return reversed(output) if len(output) == numCourses else []