CRF（条件随机场，Conditional Random Field）
- 用简单的语言理解CRF https://www.sohu.com/a/207085690_206784
- 以例子的方式来说明CRF https://www.imooc.com/article/27795
- 详细的阐述 https://zhuanlan.zhihu.com/P/28465510
- An Introduction to Conditional Random Fields http://homepages.inf.ed.ac.uk/csutton/publications/crftut-fnt.pdf