Refine kotlin code (#1241)

* style(kotlin): Improve kotlin codes readability.

* remove redundant quotes.

* style(kotlin): improve codes readability.

* style(kotlin): refine kotlin codes.

* Create kotlin.yml

* Create kotlin.yml

* Delete .github/workflows/kotlin

* Delete .github/workflows/main.yml

* Create kotlin.yml

* Update kotlin.yml

* Delete .github/workflows/kotlin.yml

* Create hello_world_workflow.main.kts

* Delete .github/workflows/hello_world_workflow.main.kts

* remove empty line

codes/kotlin/utils/TreeNode.kt

@@ -7,6 +7,7 @@
 package utils
 
 /* 二叉树节点类 */
+/* 构造方法 */
 class TreeNode(
     var value: Int // 节点值
 ) {
@@ -59,8 +60,8 @@ class TreeNode(
         }
 
         /* 将二叉树序列化为列表 */
-        fun treeToList(root: TreeNode?): List<Int?> {
-            val res = ArrayList<Int?>()
+        fun treeToList(root: TreeNode?): MutableList<Int?> {
+            val res = mutableListOf<Int?>()
             treeToListDFS(root, 0, res)
             return res
         }