Update the logo and favicon (#924)

* Update ieration_and_recursion.md

* Update the logo and favicon.

docs/chapter_computational_complexity/iteration_and_recursion.md

@@ -1,6 +1,6 @@
 # 迭代与递归
 
-在数据结构与算法中，重复执行某个任务是很常见的，其与算法的复杂度密切相关。而要重复执行某个任务，我们通常会选用两种基本的程序结构：迭代和递归。
+在算法中，重复执行某个任务是很常见的，其与复杂度分析息息相关。因此，在展开介绍时间复杂度和空间复杂度之前，我们先来了解如何在程序中实现重复执行任务，即两种基本的程序控制结构：迭代、递归。
 
 ## 迭代
 

mkdocs.yml

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