泊松分布和指数分布：10分钟教程

http://www.ruanyifeng.com/blog/2015/06/poisson-distribution.html

https://www.zhihu.com/question/24796044

https://www.cnblogs.com/think-and-do/p/6483335.html

https://www.cnblogs.com/Mr-ZeroW/p/7688436.html