简单聚合：输入单个Series，输出只有一个元素的Series。 复杂聚合：输入多个Series，输出多行和多列。 复杂聚合需要自定义函数计算出我们想要的结果。

A good cordless stick vacuum cleaner gives you the freedom to move from room to room without dragging a cable or hunting for the nearest outlet. They’re lighter than a typical corded vacuum and often ...