Counter类

介绍:A counter tool is provided to support convenient and rapid tallies

构造：class collections.Counter([iterable-or-mapping])

是dict的子类：取：c['cats']，返回个数值；赋：c['cats'] = 0

方法：

elements()返回元素

most_common([n])

subtract([iterable-or-mapping])

fromkeys(iterable)

update([iterable-or-mapping])

文档：

Python中Collections模块的Counter容器类使用教程

counter类官网https://docs.python.org/3.5/library/collections.html#collections.Counter

---