上次参加 leetcode 比赛还是去年四月份的时候了。回想的时候总觉得时间过得很快。只是一年，以前一起比赛的小伙伴们包括我自己都已经离开哥村了。

第一题Convert Binary Number in a Linked List to Integer

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def getDecimalValue(self, head: ListNode) -> int:
        r = 0
        while head:
            r = r * 2 + head.val
            head = head.next
        return r

这篇笔记尝试着实现几种简单的机遇梯度下降的优化器 (Optimizer)。主要的参考资料是 Neural Networks Part 3: Learning and Evaluation。另外我还参考了 An overview of gradient descent optimization algorithms。这篇博文不仅有篇对应的论文：An overview of gradient descent optimization algorithms，还已经被翻译成中文了：梯度下降优化算法综述。

逻辑回归 (logistic regression) 是广义线性模型 (generalized linear model) 的一种，对应的是伯努利分布 (Bernoulli distribution)。如果我们在广义线性模型中使用的是多项分布 (multinomial distribution)，那么我们将得到逻辑回归的推广——多项逻辑回归 (multinomial logistic regression)。我们在这里用逻辑回归来代表逻辑回归和多项逻辑回归。

我们现在已经知道机器学习三个重要的模块是模型 (model)、标准 (criterion) 以及 优化 (optimization)。在这篇笔记中我们讨论各个模块必要的基本功能，然后用 Python 给出模块的基类 (base class)。 最后我们继承这些基类实现上一篇笔记的线性模型 (linear model) 以及均方误差 (mean square error)。

很早之前我就有一个“写一个机器学习的系列笔记”的想法了。之所以叫做“笔记”而不叫“教程”，是因为机器学习只是我的业余爱好之一，我并不精通此道。事实上，我只是学到最简单的皮毛知识，还不一定明白透彻。所以我把我将要写的这个系列定义为笔记，并且在开篇就写得明明白白，这样我就可以随心所欲地写而并不担心误人子弟。不过既然我把笔记放在网上了，那表明我也希望其他人能从我写的东西里得到启发。如果你发现我的理解有误或者文章有错，也请务必留言让我知道。

这个笔记记录如何在 Raspbian 上编辑 Raspbian 镜像。为什么会有这个需要呢？第一是因为经常在 Raspberry Pi 上尝试各种各样的东西，会把系统弄得乱七八糟，所以经常要重新刷 Raspbian ，因此就有了编辑 Raspbian 镜像的需求。第二是我现在用的是 Mac OS ，编辑镜像文件并不那么方便，所以就干脆在 Raspbian 系统中编辑 Raspbian 镜像了。

2018年，一直在路上，特别是在I-71。I-71是一条跨州的高速公路，从南端肯塔基州的路易斯维尔(Louisville, Kentucky)东北延向俄亥俄州的克里夫兰(Cleveland, Ohio)，全长约345迈。之前每逢夏天，便约上几个好友沿着I-71开车到克里夫兰，或是参观艺术博物馆，或是听音乐会，又或是去那个不甚似国家公园的国家公园骑自行车。到了今年，那些个好友也已经有好几个毕业离开了。虽然少了好友一起在I-71路上寻找悠闲，但是I-71对我来说有了它新的意义：它变成了李之仪的长江。

卜算子

我住长江尾，君住长江尾。
日日思君不见君，共饮长江水。
此水几时休，此恨何时已。
只愿君心似我心，定不负相思意。

Sudoku is a very interesting puzzle, and it is very easy to get familiar with the rules of the game. I always like to compare sudoku with number theory, they are easy to be understood but can be hard to solve. Fermat's Last Theorem, an easily understood problem even by middle school students, took mathematians 358 years to solve it completely. Solving sudoku puzzles can take even longer: it is proven that solving sudoku puzzels is an NP-complete problem! However, classical 9x9 sudoku puzzles are still feasible using backtracking algorithm. We are not going to talk about how to solve sudoku puzzles here though, we are trying to sovle another problem: detect grids in images containing sudoku puzzles.

To make the problem more concrete, let's make some assumption on the image that contains a sudoku puzzle:

1. The image only contains one sudoku puzzle
2. The sudoku puzzle is the largest block in the image
3. There is no/little distorsion/rotation of the sudoku puzzle grid

Also in this article, we focus on only simple solutions without heavy image processing. We will use Pillow to read images and convert them to gray scale images, and we use Numpy to manipulate images as arrays. For visualization, we use Matplotlib.

Finally, I successfully installed Pytorch on my RaspberryPi 3B. I followed the instructions on How to install PyTorch v0.3.1 on RaspberryPi 3B and a blog post (in Chinese) 在 RaspberryPi 上编译 PyTorch. I am running latest raspian image 2018-04-18-raspbian-stretch and a self-compiled Python 3.6.5. If you are interested in compile Python 3.6.5 or above on your own, check out Installing Python 3.6 on Raspbian. In this post, I will write down steps to my successful installation, and share my compiled wheel file at the end.

I recently built a flask app that run on my raspberry pi. Then I turned it down. Why? In order to access the app from outside the home internet, I had to expose my pi to public. After only one day of exposure, my pi received several attack attempts and the flask app then always replied 301. Here are some attack logs:

139.162.83.10 - "GET / HTTP/1.1" 404 -
45.33.115.189 - "GET / HTTP/1.0" 404 -
79.55.11.21 - "GET / HTTP/1.0" 404 -
5.101.40.78 - code 400, message Bad HTTP/0.9 request type ('\x03\x00\x00/*à\x00\x00\x00\x00\x00Cookie:')
5.101.40.78 - "/*àCookie: mstshash=Administr" HTTPStatus.BAD_REQUEST -
123.151.42.61 - "GET http://www.baidu.com/ HTTP/1.1" 404 -
158.85.81.116 - "GET / HTTP/1.1" 404 -

To be more precise, the flask app was still running, but the home router seemed not forwarding the requests to my pi. Anyway, I have no idea of what have happened. I just shut the app down, flashed a fresh new image to my pi.

I want a way to communicate with my pi without exposing it. That is the motivation I start the project chapi, which means "chat with pi".