210

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

        g = dict()

        for i in range(numCourses):
            g[i] = []

        for a, b in prerequisites:
            g[b].append(a)

        def toposort(v, flag):
            if not flag:
                return flag
            visited[v] = 1
            for n in g[v]:
                if not flag:
                    return flag
                if visited[n] == 0:
                    flag = toposort(n, flag)
                elif visited[n] == 1:
                    flag = False

            visited[v] = 2
            st.append(v)
            return flag

        visited = [0] * numCourses
        st = []
        flag = True
        for v in g:
            if not visited[v] and flag:
                flag = toposort(v, flag)

        return st[::-1] if flag else []