同样是拓扑排序，需要注意的是防治出现闭环的情况。当出现闭环，返回空。
python 代码：
class Solution:
"""
@param: numCourses: a total of n courses
@param: prerequisites: a list of prerequisite pairs
@return: the course order
"""
def findOrder(self, numCourses, prerequisites):
# write your code here
from collections import deque
edge = [[] for i in range(numCourses)]
indegree = [0] * numCourses
for i, j in prerequisites:
edge[j].append(i)
indegree[i] += 1
res = []
queue = deque()
cnt = 0
for i in range(numCourses):
if indegree[i] == 0:
queue.append(i)
while queue:
course = queue.popleft()
res.append(course)
cnt += 1
for neighbor in edge[course]:
indegree[neighbor] -= 1
if indegree[neighbor] == 0:
queue.append(neighbor)
if cnt != numCourses:
return []
return res
