https://en.wikipedia.org/wiki/Maximum_subarray_problem#Algorithm_1:_Brute_Force
http://blog.youkuaiyun.com/lengxiao1993/article/details/52303492
https://en.wikipedia.org/wiki/Maximum_subarray_problem#Algorithm_1:_Brute_Force
http://blog.youkuaiyun.com/lengxiao1993/article/details/52303492