* Sync zh and zh-hant versions.
* Remove the polyfill.io link from mkdocs.yml
* Update contributors' info for code reviewers and en/zh-hant versions reviewers.
* Fix graph.md
* Update avatars for English version reviewers.
* Sync zh and zh-hant versions.
* Fix two_sum_brute_force.png
* Sync zh and zh-hant versions.
Optimize structrue of index.html.
* Format index.html
* Revised the figures thoughout the book.
* Utilize roboto and roboto mono as the font faimly of the English edition
* Fix the naming style
* Update two figures.
* Fix margin of the hero image
* Fix margin of the hero image
* Update all the figures
* Update the data structure, sorting, divide and conquer chapters' covers. Add the English edition covers.
* Fix the link of EN translation in README
* Update README.md
* Update README.md
* Update README-en.md