from collections import defaultdict 
 
class Graph: 
 def __init__(self,vertices): 
  self.graph = defaultdict(list) 
  self.V = vertices
 
 def addEdge(self,u,v): 
  self.graph[u].append(v) 
 
 def topologicalSortUtil(self,v,visited,stack): 
 
  visited[v] = True
 
  for i in self.graph[v]: 
   if visited[i] == False: 
    self.topologicalSortUtil(i,visited,stack) 
 
  stack.insert(0,v) 
 
 def topologicalSort(self): 
  visited = [False]*self.V 
  stack =[] 
 
  for i in range(self.V): 
   if visited[i] == False: 
    self.topologicalSortUtil(i,visited,stack) 
 
  print (stack) 
 
g= Graph(6) 
g.addEdge(5, 2); 
g.addEdge(5, 0); 
g.addEdge(4, 0); 
g.addEdge(4, 1); 
g.addEdge(2, 3); 
g.addEdge(3, 1); 
 
print ("拓扑排序结果：")
g.topologicalSort()

def toposort(graph):
 in_degrees = dict((u,0) for u in graph) #初始化所有顶点入度为0
 vertex_num = len(in_degrees)
 for u in graph:
  for v in graph[u]:
   in_degrees[v] += 1  #计算每个顶点的入度
 Q = [u for u in in_degrees if in_degrees[u] == 0] # 筛选入度为0的顶点
 Seq = []
 while Q:
  u = Q.pop()  #默认从最后一个删除
  Seq.append(u)
  for v in graph[u]:
   in_degrees[v] -= 1  #移除其所有指向
   if in_degrees[v] == 0:
    Q.append(v)   #再次筛选入度为0的顶点
 if len(Seq) == vertex_num:  #如果循环结束后存在非0入度的顶点说明图中有环，不存在拓扑排序
  return Seq
 else:
  print("there's a circle.")
G = {
 'a':'bce',
 'b':'d',
 'c':'d',
 'd':'',
 'e':'cd'
}
print(toposort(G))