面试被问到了，没做出来，以后不能再有侥幸心理了。

class Solution(object):

def canFinish(self, numCourses, prerequisites):

dict_graph={}

for i in range(numCourses):

dict_graph[i]={}

for prereq in prerequisites:

dict_graph[prereq[0]][prereq[1]]=1

def del0indegree(dict_graph):

tobedel=[]

for node in dict_graph:

if len(dict_graph[node])==0:

tobedel.append(node)

for tobedel_i in tobedel:

del dict_graph[tobedel_i]

for node in dict_graph:

for tobedel_i in tobedel:

if tobedel_i in dict_graph[node]:

del dict_graph[node][tobedel_i]

def has0indegree(dict_graph):

for node in dict_graph:

if len(dict_graph[node].keys())==0:

return True

return False

while has0indegree(dict_graph):

del0indegree(dict_graph)

return True if len(dict_graph.keys())==0 else False