Review Swift codes (#1150)

* feat(swift): review for chapter_computational_complexity

* feat(swift): review for chapter_data_structure

* feat(swift): review for chapter_array_and_linkedlist

* feat(swift): review for chapter_stack_and_queue

* feat(swift): review for chapter_hashing

* feat(swift): review for chapter_tree

* feat(swift): add codes for heap article

* feat(swift): review for chapter_heap

* feat(swift): review for chapter_graph

* feat(swift): review for chapter_searching

* feat(swift): review for chapter_sorting

* feat(swift): review for chapter_divide_and_conquer

* feat(swift): review for chapter_backtracking

* feat(swift): review for chapter_dynamic_programming

* feat(swift): review for chapter_greedy

* feat(swift): review for utils

* feat(swift): update ci tool

* feat(swift): trailing closure

* feat(swift): array init

* feat(swift): map index

codes/swift/chapter_tree/avl_tree.swift

@@ -10,10 +10,12 @@ import utils
 class AVLTree {
     fileprivate var root: TreeNode? // 根节点
 
+    init() {}
+
     /* 获取节点高度 */
     func height(node: TreeNode?) -> Int {
         // 空节点高度为 -1 ，叶节点高度为 0
-        node == nil ? -1 : node!.height
+        node?.height ?? -1
     }
 
     /* 更新节点高度 */
@@ -132,7 +134,7 @@ class AVLTree {
             node?.right = removeHelper(node: node?.right, val: val)
         } else {
             if node?.left == nil || node?.right == nil {
-                let child = node?.left != nil ? node?.left : node?.right
+                let child = node?.left ?? node?.right
                 // 子节点数量 = 0 ，直接删除 node 并返回
                 if child == nil {
                     return nil