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)