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/subset_sum_i.kt

@@ -6,19 +6,17 @@
 
 package chapter_backtracking.subset_sum_i
 
-import java.util.*
-
 /* 回溯算法：子集和 I */
 fun backtrack(
     state: MutableList<Int>,
     target: Int,
     choices: IntArray,
     start: Int,
-    res: MutableList<List<Int>?>
+    res: MutableList<MutableList<Int>?>
 ) {
     // 子集和等于 target 时，记录解
     if (target == 0) {
-        res.add(ArrayList(state))
+        res.add(state.toMutableList())
         return
     }
     // 遍历所有选择
@@ -39,11 +37,11 @@ fun backtrack(
 }
 
 /* 求解子集和 I */
-fun subsetSumI(nums: IntArray, target: Int): List<List<Int>?> {
-    val state: MutableList<Int> = ArrayList() // 状态（子集）
-    Arrays.sort(nums) // 对 nums 进行排序
+fun subsetSumI(nums: IntArray, target: Int): MutableList<MutableList<Int>?> {
+    val state = mutableListOf<Int>() // 状态（子集）
+    nums.sort() // 对 nums 进行排序
     val start = 0 // 遍历起始点
-    val res: MutableList<List<Int>?> = ArrayList() // 结果列表（子集列表）
+    val res = mutableListOf<MutableList<Int>?>() // 结果列表（子集列表）
     backtrack(state, target, nums, start, res)
     return res
 }
@@ -57,4 +55,4 @@ fun main() {
 
     println("输入数组 nums = ${nums.contentToString()}, target = $target")
     println("所有和等于 $target 的子集 res = $res")
-}
+}