WARNNING!!!
For learning purposes only, please don't copy my code directly. I'm not responsible for your essential plagiarism.
NOTE
If you CANNOT see the math formulas (instead, a lot of messy codes), please use Chrome and go to this cite:
<chrome.google.com/webstore/detail/mathjax-plugin-for-github/ioemnmodlmafdkllaclgeombjnmnbima/related>
to download the extension to view the formulas.