Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.
定义一组算法，将每个算法都封装起来，并且使它们之间可以互换

The classes and objects participating in this pattern are:

• declares an interface common to all supported algorithms. Context uses this interface to call the algorithm defined by a ConcreteStrategy

• implements the algorithm using the Strategy interface

• is configured with a ConcreteStrategy object
• maintains a reference to a Strategy object
• may define an interface that lets Strategy access its data.