说明：  模仿标准库的vector 模板，实现一个可以存储一系列同类型数据元素的向量类Vector. 实现常用的一些操作，如下标运算符[], push_bck(), size()等。
(Vector template to mimic the standard library, the realization of a series of class Vector can store vector data elements of the same type. Achieve some common operations, as subscript operator [], push_bck (), size () and so on.)