今天刚看到这一个示例，但是运行时结果中出现None。改了一下格式，出现了正确结果：
states_needed=set(['mt','wa','or','id','nv','ut','ca','az']) #传入一个数组，它被转换为集合 州
stations={} #广播台清单
stations['kone']=set(['id','nv','ut'])
stations['ktwo']=set(['wa','id','mt'])
stations['kthree']=set(['or','nv','ca'])
stations['kfour']=set(['nv','ut'])
stations['kfive']=set(['ca','az'])
final_station=set() #最终选择的广播台
while states_needed:
best_station=None
states_covered=set()
for station,states_for_station in stations.items():
'''
print(station)
print('*station*')
print(states_for_station)
print('*states_for_station*')
'''
covered=states_needed & states_for_station #计算交集
if len(covered) > len(states_covered):
best_station=station
states_covered=covered
#这个地方书上的格式有问题
states_needed-=states_covered
final_station.add(best_station)
print(final_station)
明天仔细看这个案例。