Abstract grade util for grading a common List to a single, double or multiple Tree Structure List in which some items have parents.
This blog will show you how it helps data binding to views.
https://my.oschina.net/tommylemon/blog/885787
抽象列表分级工具，支持微信朋友圈单层评论、QQ空间双层评论、百度网盘多层(无限层)文件夹等。
效果见 仿QQ空间和微信朋友圈，高解耦高复用高灵活