greedy算法的核心思想是首先计算覆盖面大的部分，然后依次寻找其他覆盖面最大的部分。该算法的使用场景就像他的名字一样，当符合贪婪属性的时候就可以考虑。
states_needed = set(['北京', '上海', '广州', '深圳', '杭州', '南京', '石家庄', '银川'])
stations = {}
stations['kone'] = set(['北京', '上海', '广州'])
stations['ktwo'] = set(['北京''杭州', '南京'])
stations['kthree'] = set(['广州', '深圳', '杭州'])
stations['kfour'] = set(['北京', '银川'])
stations['kfive'] = set(['石家庄', '银川'])
final_stations = set()
while states_needed:
best_station = None
states_covered = set()
for station, states in stations.items():
covered = states_needed & states
if len(covered) > len(states_covered):
best_station = station
states_covered = covered
states_needed -= states_covered
final_stations.add(best_station)
print(final_stations)
