Simplify kotlin code and improve code readability (#1198)

* Add kotlin code block for chapter_hashing

* Add kotlin code block for chapter_heap.

* Add kotlin code block for chapter_stack_and_queue and chapter_tree

* fix indentation

* Update binary_tree.md

* style(kotlin): simplify code and improve readability.

* simplify kt code for chapter_computational_complexity.

* style(kotlin): replace ArrayList with MutableList.

* Update subset_sum_i.kt

Use kotlin api instead of java.

* Update subset_sum_ii.kt

use kotlin api instead of java

* style(kotlin): replace ArrayList with mutablelist.

---------

Co-authored-by: Yudong Jin <krahets@163.com>

codes/kotlin/chapter_backtracking/preorder_traversal_iii_compact.kt

@@ -10,7 +10,7 @@ import utils.TreeNode
 import utils.printTree
 
 var path: MutableList<TreeNode>? = null
-var res: MutableList<List<TreeNode>>? = null
+var res: MutableList<MutableList<TreeNode>>? = null
 
 /* 前序遍历：例题三 */
 fun preOrder(root: TreeNode?) {
@@ -22,7 +22,7 @@ fun preOrder(root: TreeNode?) {
     path!!.add(root)
     if (root.value == 7) {
         // 记录解
-        res!!.add(ArrayList(path!!))
+        res!!.add(path!!.toMutableList())
     }
     preOrder(root.left)
     preOrder(root.right)
@@ -37,13 +37,13 @@ fun main() {
     printTree(root)
 
     // 前序遍历
-    path = ArrayList()
-    res = ArrayList()
+    path = mutableListOf()
+    res = mutableListOf()
     preOrder(root)
 
     println("\n输出所有根节点到节点 7 的路径，路径中不包含值为 3 的节点")
-    for (path in res as ArrayList<List<TreeNode>>) {
-        val values: MutableList<Int> = ArrayList()
+    for (path in res!!) {
+        val values = mutableListOf<Int>()
         for (node in path) {
             values.add(node.value)
         }