容器介绍－－NSArray,NSSet,NSDictionaryhttp://liwpk.blog.163.com/blog/static/363261702011472318549/htt
容器介绍
－－NSArray,NSSet,NSDictionary
http://liwpk.blog.163.com/blog/static/363261702011472318549/
http://my.oschina.net/ahuaahua/blog/14554
Mutab，使得容器可以进行增删改，经常结合NSNull使用