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.

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

codes/kotlin/chapter_computational_complexity/space_complexity.kt

@@ -60,9 +60,9 @@ fun linearRecur(n: Int) {
 /* 平方阶 */
 fun quadratic(n: Int) {
     // 矩阵占用 O(n^2) 空间
-    val numMatrix: Array<Array<Int>?> = arrayOfNulls(n)
+    val numMatrix = arrayOfNulls<Array<Int>?>(n)
     // 二维列表占用 O(n^2) 空间
-    val numList: MutableList<MutableList<Int>> = arrayListOf()
+    val numList = mutableListOf<MutableList<Int>>()
     for (i in 0..<n) {
         val tmp = mutableListOf<Int>()
         for (j in 0..<n) {
@@ -104,6 +104,6 @@ fun main() {
     quadratic(n)
     quadraticRecur(n)
     // 指数阶
-    val root: TreeNode? = buildTree(n)
+    val root = buildTree(n)
     printTree(root)
 }