Bug fixes and improvements (#1348)

* Add "reference" for EN version. Bug fixes.

* Unify the figure reference as "the figure below" and "the figure above".
Bug fixes.

* Format the EN markdown files.

* Replace "" with <u></u> for EN version and bug fixes

* Fix biary_tree_dfs.png

* Fix biary_tree_dfs.png

* Fix zh-hant/biary_tree_dfs.png

* Fix heap_sort_step1.png

* Sync zh and zh-hant versions.

* Bug fixes

* Fix EN figures

* Bug fixes

* Fix the figure labels for EN version

codes/ruby/chapter_sorting/quick_sort.rb

@@ -9,7 +9,6 @@ class QuickSort
   class << self
     ### 哨兵划分 ###
     def partition(nums, left, right)
-
       # 以 nums[left] 为基准数
       i, j = left, right
       while i < j
@@ -116,7 +115,7 @@ class QuickSortTailCall
       i # 返回基准数的索引
     end
 
-    ### 快速排序（尾递归优化）
+    ### 快速排序（尾递归优化）###
     def quick_sort(nums, left, right)
       # 子数组长度不为 1 时递归
       while left < right