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/chapter_tree/binary_tree_bfs.kt

@@ -16,13 +16,14 @@ fun levelOrder(root: TreeNode?): MutableList<Int> {
     val queue = LinkedList<TreeNode?>()
     queue.add(root)
     // 初始化一个列表，用于保存遍历序列
-    val list = ArrayList<Int>()
-    while (!queue.isEmpty()) {
-        val node = queue.poll() // 队列出队
-        list.add(node?.value!!) // 保存节点值
-        if (node.left != null) queue.offer(node.left) // 左子节点入队
-
-        if (node.right != null) queue.offer(node.right) // 右子节点入队
+    val list = mutableListOf<Int>()
+    while (queue.isNotEmpty()) {
+        val node = queue.poll()      // 队列出队
+        list.add(node?.value!!)      // 保存节点值
+        if (node.left != null)
+            queue.offer(node.left)   // 左子节点入队
+        if (node.right != null)
+            queue.offer(node.right)  // 右子节点入队
     }
     return list
 }
@@ -38,4 +39,4 @@ fun main() {
     /* 层序遍历 */
     val list = levelOrder(root)
     println("\n层序遍历的节点打印序列 = $list")
-}
+}