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_array_and_linkedlist/array.kt

@@ -55,7 +55,7 @@ fun traverse(nums: IntArray) {
         count += nums[i]
     }
     // 直接遍历数组元素
-    for (j: Int in nums) {
+    for (j in nums) {
         count += j
     }
 }
@@ -63,7 +63,8 @@ fun traverse(nums: IntArray) {
 /* 在数组中查找指定元素 */
 fun find(nums: IntArray, target: Int): Int {
     for (i in nums.indices) {
-        if (nums[i] == target) return i
+        if (nums[i] == target)
+            return i
     }
     return -1
 }

codes/kotlin/chapter_array_and_linkedlist/linked_list.kt

@@ -9,7 +9,7 @@ package chapter_array_and_linkedlist
 import utils.ListNode
 import utils.printLinkedList
 
-/* 在链表的节点 n0 之后插入节点p */
+/* 在链表的节点 n0 之后插入节点 P */
 fun insert(n0: ListNode?, p: ListNode?) {
     val n1 = n0?.next
     p?.next = n1
@@ -18,16 +18,20 @@ fun insert(n0: ListNode?, p: ListNode?) {
 
 /* 删除链表的节点 n0 之后的首个节点 */
 fun remove(n0: ListNode?) {
-    val p = n0?.next
+    if (n0?.next == null)
+        return
+    val p = n0.next
     val n1 = p?.next
-    n0?.next = n1
+    n0.next = n1
 }
 
 /* 访问链表中索引为 index 的节点 */
 fun access(head: ListNode?, index: Int): ListNode? {
     var h = head
     for (i in 0..<index) {
-        h = h?.next
+        if (h == null)
+            return null
+        h = h.next
     }
     return h
 }
@@ -37,7 +41,8 @@ fun find(head: ListNode?, target: Int): Int {
     var index = 0
     var h = head
     while (h != null) {
-        if (h.value == target) return index
+        if (h.value == target)
+            return index
         h = h.next
         index++
     }
@@ -46,6 +51,7 @@ fun find(head: ListNode?, target: Int): Int {
 
 /* Driver Code */
 fun main() {
+    /* 初始化链表 */
     // 初始化各个节点
     val n0 = ListNode(1)
     val n1 = ListNode(3)
@@ -60,7 +66,8 @@ fun main() {
     n3.next = n4
     println("初始化的链表为")
     printLinkedList(n0)
-    
+
+    /* 插入节点 */
     insert(n0, ListNode(0))
     println("插入节点后的链表为")
     printLinkedList(n0)

codes/kotlin/chapter_array_and_linkedlist/list.kt

@@ -8,9 +8,9 @@ package chapter_array_and_linkedlist
 
 /* Driver Code */
 fun main() {
+    /* 初始化列表 */
     // 可变集合
-    val numbers = mutableListOf(1, 3, 2, 5, 4)
-    val nums = ArrayList<Int>(numbers)
+    val nums = mutableListOf(1, 3, 2, 5, 4)
     println("列表 nums = $nums")
 
     /* 访问元素 */
@@ -53,11 +53,11 @@ fun main() {
     }
 
     /* 拼接两个列表*/
-    val nums1 = ArrayList<Int>(listOf(6, 8, 7, 10, 9))
+    val nums1 = mutableListOf(6, 8, 7, 10, 9)
     nums.addAll(nums1)
     println("将列表 nums1 拼接到 nums 之后，得到 nums = $nums")
 
     /* 排序列表 */
-    nums.sort() //排序后,列表元素从小到大排列
+    nums.sort()
     println("排序列表后 nums = $nums")
 }

codes/kotlin/chapter_array_and_linkedlist/my_list.kt

@@ -9,11 +9,11 @@ package chapter_array_and_linkedlist
 /* 列表类 */
 class MyList {
     private var arr: IntArray = intArrayOf() // 数组（存储列表元素）
-    private var capacity = 10 // 列表容量
-    private var size = 0 // 列表长度（当前元素数量）
-    private var extendRatio = 2 // 每次列表扩容的倍数
+    private var capacity: Int = 10 // 列表容量
+    private var size: Int = 0 // 列表长度（当前元素数量）
+    private var extendRatio: Int = 2 // 每次列表扩容的倍数
 
-    /* 构造函数 */
+    /* 构造方法 */
     init {
         arr = IntArray(capacity)
     }
@@ -32,7 +32,7 @@ class MyList {
     fun get(index: Int): Int {
         // 索引如果越界，则抛出异常，下同
         if (index < 0 || index >= size)
-            throw IndexOutOfBoundsException()
+            throw IndexOutOfBoundsException("索引越界")
         return arr[index]
     }
 
@@ -72,7 +72,7 @@ class MyList {
     fun remove(index: Int): Int {
         if (index < 0 || index >= size)
             throw IndexOutOfBoundsException("索引越界")
-        val num: Int = arr[index]
+        val num = arr[index]
         // 将将索引 index 之后的元素都向前移动一位
         for (j in index..<size - 1)
             arr[j] = arr[j + 1]

codes/kotlin/chapter_backtracking/n_queens.kt

@@ -10,17 +10,17 @@ package chapter_backtracking.n_queens
 fun backtrack(
     row: Int,
     n: Int,
-    state: List<MutableList<String>>,
-    res: MutableList<List<List<String>>?>,
+    state: MutableList<MutableList<String>>,
+    res: MutableList<MutableList<MutableList<String>>?>,
     cols: BooleanArray,
     diags1: BooleanArray,
     diags2: BooleanArray
 ) {
     // 当放置完所有行时，记录解
     if (row == n) {
-        val copyState: MutableList<List<String>> = ArrayList()
+        val copyState = mutableListOf<MutableList<String>>()
         for (sRow in state) {
-            copyState.add(ArrayList(sRow))
+            copyState.add(sRow.toMutableList())
         }
         res.add(copyState)
         return
@@ -49,11 +49,11 @@ fun backtrack(
 }
 
 /* 求解 n 皇后 */
-fun nQueens(n: Int): List<List<List<String>>?> {
+fun nQueens(n: Int): MutableList<MutableList<MutableList<String>>?> {
     // 初始化 n*n 大小的棋盘，其中 'Q' 代表皇后，'#' 代表空位
-    val state: MutableList<MutableList<String>> = ArrayList()
+    val state = mutableListOf<MutableList<String>>()
     for (i in 0..<n) {
-        val row: MutableList<String> = ArrayList()
+        val row = mutableListOf<String>()
         for (j in 0..<n) {
             row.add("#")
         }
@@ -62,7 +62,7 @@ fun nQueens(n: Int): List<List<List<String>>?> {
     val cols = BooleanArray(n) // 记录列是否有皇后
     val diags1 = BooleanArray(2 * n - 1) // 记录主对角线上是否有皇后
     val diags2 = BooleanArray(2 * n - 1) // 记录次对角线上是否有皇后
-    val res: MutableList<List<List<String>>?> = ArrayList()
+    val res = mutableListOf<MutableList<MutableList<String>>?>()
 
     backtrack(0, n, state, res, cols, diags1, diags2)
 
@@ -72,7 +72,7 @@ fun nQueens(n: Int): List<List<List<String>>?> {
 /* Driver Code */
 fun main() {
     val n = 4
-    val res: List<List<List<String?>?>?> = nQueens(n)
+    val res = nQueens(n)
 
     println("输入棋盘长宽为 $n")
     println("皇后放置方案共有 ${res.size} 种")

codes/kotlin/chapter_backtracking/permutations_i.kt

@@ -11,11 +11,11 @@ fun backtrack(
     state: MutableList<Int>,
     choices: IntArray,
     selected: BooleanArray,
-    res: MutableList<List<Int>?>
+    res: MutableList<MutableList<Int>?>
 ) {
     // 当状态长度等于元素数量时，记录解
     if (state.size == choices.size) {
-        res.add(ArrayList(state))
+        res.add(state.toMutableList())
         return
     }
     // 遍历所有选择
@@ -36,9 +36,9 @@ fun backtrack(
 }
 
 /* 全排列 I */
-fun permutationsI(nums: IntArray): List<List<Int>?> {
-    val res: MutableList<List<Int>?> = ArrayList()
-    backtrack(ArrayList(), nums, BooleanArray(nums.size), res)
+fun permutationsI(nums: IntArray): MutableList<MutableList<Int>?> {
+    val res = mutableListOf<MutableList<Int>?>()
+    backtrack(mutableListOf(), nums, BooleanArray(nums.size), res)
     return res
 }
 

codes/kotlin/chapter_backtracking/permutations_ii.kt

@@ -15,11 +15,11 @@ fun backtrack(
 ) {
     // 当状态长度等于元素数量时，记录解
     if (state.size == choices.size) {
-        res.add(ArrayList(state))
+        res.add(state.toMutableList())
         return
     }
     // 遍历所有选择
-    val duplicated: MutableSet<Int> = HashSet()
+    val duplicated = HashSet<Int>()
     for (i in choices.indices) {
         val choice = choices[i]
         // 剪枝：不允许重复选择元素 且 不允许重复选择相等元素
@@ -39,15 +39,14 @@ fun backtrack(
 
 /* 全排列 II */
 fun permutationsII(nums: IntArray): MutableList<MutableList<Int>?> {
-    val res: MutableList<MutableList<Int>?> = ArrayList()
-    backtrack(ArrayList(), nums, BooleanArray(nums.size), res)
+    val res = mutableListOf<MutableList<Int>?>()
+    backtrack(mutableListOf(), nums, BooleanArray(nums.size), res)
     return res
 }
 
 /* Driver Code */
 fun main() {
     val nums = intArrayOf(1, 2, 2)
-
     val res = permutationsII(nums)
 
     println("输入数组 nums = ${nums.contentToString()}")

codes/kotlin/chapter_backtracking/preorder_traversal_i_compact.kt

@@ -31,12 +31,12 @@ fun main() {
     printTree(root)
 
     // 前序遍历
-    res = ArrayList()
+    res = mutableListOf()
     preOrder(root)
 
     println("\n输出所有值为 7 的节点")
-    val vals: MutableList<Int> = ArrayList()
-    for (node in res as ArrayList<TreeNode>) {
+    val vals = mutableListOf<Int>()
+    for (node in res!!) {
         vals.add(node.value)
     }
     println(vals)

codes/kotlin/chapter_backtracking/preorder_traversal_ii_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：例题二 */
 fun preOrder(root: TreeNode?) {
@@ -21,7 +21,7 @@ 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)
@@ -36,13 +36,13 @@ fun main() {
     printTree(root)
 
     // 前序遍历
-    path = java.util.ArrayList<TreeNode>()
-    res = java.util.ArrayList<List<TreeNode>>()
+    path = mutableListOf()
+    res = mutableListOf()
     preOrder(root)
 
     println("\n输出所有根节点到节点 7 的路径")
-    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)
         }

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)
         }

codes/kotlin/chapter_backtracking/preorder_traversal_iii_template.kt

@@ -8,20 +8,19 @@ package chapter_backtracking.preorder_traversal_iii_template
 
 import utils.TreeNode
 import utils.printTree
-import java.util.*
 
 /* 判断当前状态是否为解 */
-fun isSolution(state: List<TreeNode?>): Boolean {
+fun isSolution(state: MutableList<TreeNode?>): Boolean {
     return state.isNotEmpty() && state[state.size - 1]?.value == 7
 }
 
 /* 记录解 */
-fun recordSolution(state: MutableList<TreeNode?>?, res: MutableList<List<TreeNode?>?>) {
-    res.add(state?.let { ArrayList(it) })
+fun recordSolution(state: MutableList<TreeNode?>?, res: MutableList<MutableList<TreeNode?>?>) {
+    res.add(state!!.toMutableList())
 }
 
 /* 判断在当前状态下，该选择是否合法 */
-fun isValid(state: List<TreeNode?>?, choice: TreeNode?): Boolean {
+fun isValid(state: MutableList<TreeNode?>?, choice: TreeNode?): Boolean {
     return choice != null && choice.value != 3
 }
 
@@ -38,8 +37,8 @@ fun undoChoice(state: MutableList<TreeNode?>, choice: TreeNode?) {
 /* 回溯算法：例题三 */
 fun backtrack(
     state: MutableList<TreeNode?>,
-    choices: List<TreeNode?>,
-    res: MutableList<List<TreeNode?>?>
+    choices: MutableList<TreeNode?>,
+    res: MutableList<MutableList<TreeNode?>?>
 ) {
     // 检查是否为解
     if (isSolution(state)) {
@@ -53,7 +52,7 @@ fun backtrack(
             // 尝试：做出选择，更新状态
             makeChoice(state, choice)
             // 进行下一轮选择
-            backtrack(state, listOf(choice!!.left, choice.right), res)
+            backtrack(state, mutableListOf(choice!!.left, choice.right), res)
             // 回退：撤销选择，恢复到之前的状态
             undoChoice(state, choice)
         }
@@ -67,12 +66,12 @@ fun main() {
     printTree(root)
 
     // 回溯算法
-    val res: MutableList<List<TreeNode?>?> = ArrayList()
-    backtrack(ArrayList(), mutableListOf(root), res)
+    val res = mutableListOf<MutableList<TreeNode?>?>()
+    backtrack(mutableListOf(), mutableListOf(root), res)
 
     println("\n输出所有根节点到节点 7 的路径，要求路径中不包含值为 3 的节点")
     for (path in res) {
-        val vals = ArrayList<Int>()
+        val vals = mutableListOf<Int>()
         for (node in path!!) {
             if (node != null) {
                 vals.add(node.value)

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")
-}
+}

codes/kotlin/chapter_backtracking/subset_sum_i_naive.kt

@@ -12,11 +12,11 @@ fun backtrack(
     target: Int,
     total: Int,
     choices: IntArray,
-    res: MutableList<List<Int>?>
+    res: MutableList<MutableList<Int>?>
 ) {
     // 子集和等于 target 时，记录解
     if (total == target) {
-        res.add(ArrayList(state))
+        res.add(state.toMutableList())
         return
     }
     // 遍历所有选择
@@ -35,10 +35,10 @@ fun backtrack(
 }
 
 /* 求解子集和 I（包含重复子集） */
-fun subsetSumINaive(nums: IntArray, target: Int): List<List<Int>?> {
-    val state: MutableList<Int> = ArrayList() // 状态（子集）
+fun subsetSumINaive(nums: IntArray, target: Int): MutableList<MutableList<Int>?> {
+    val state = mutableListOf<Int>() // 状态（子集）
     val total = 0 // 子集和
-    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, total, nums, res)
     return res
 }
@@ -47,8 +47,7 @@ fun subsetSumINaive(nums: IntArray, target: Int): List<List<Int>?> {
 fun main() {
     val nums = intArrayOf(3, 4, 5)
     val target = 9
-
-    val res: List<List<Int>?> = subsetSumINaive(nums, target)
+    val res = subsetSumINaive(nums, target)
 
     println("输入数组 nums = ${nums.contentToString()}, target = $target")
     println("所有和等于 $target 的子集 res = $res")

codes/kotlin/chapter_backtracking/subset_sum_ii.kt

@@ -6,19 +6,17 @@
 
 package chapter_backtracking.subset_sum_ii
 
-import java.util.*
-
 /* 回溯算法：子集和 II */
 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
@@ -44,11 +42,11 @@ fun backtrack(
 }
 
 /* 求解子集和 II */
-fun subsetSumII(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 subsetSumII(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
 }
@@ -57,9 +55,8 @@ fun subsetSumII(nums: IntArray, target: Int): List<List<Int>?> {
 fun main() {
     val nums = intArrayOf(4, 4, 5)
     val target = 9
-
     val res = subsetSumII(nums, target)
 
     println("输入数组 nums = ${nums.contentToString()}, target = $target")
     println("所有和等于 $target 的子集 res = $res")
-}
+}

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)
 }

codes/kotlin/chapter_computational_complexity/time_complexity.kt

@@ -9,7 +9,7 @@ package chapter_computational_complexity.time_complexity
 /* 常数阶 */
 fun constant(n: Int): Int {
     var count = 0
-    val size = 10_0000
+    val size = 100000
     for (i in 0..<size)
         count++
     return count
@@ -48,7 +48,7 @@ fun quadratic(n: Int): Int {
 
 /* 平方阶（冒泡排序） */
 fun bubbleSort(nums: IntArray): Int {
-    var count = 0
+    var count = 0 // 计数器
     // 外循环：未排序区间为 [0, i]
     for (i in nums.size - 1 downTo 1) {
         // 内循环：将未排序区间 [0, i] 中的最大元素交换至该区间的最右端
@@ -109,7 +109,7 @@ fun linearLogRecur(n: Int): Int {
     if (n <= 1)
         return 1
     var count = linearLogRecur(n / 2) + linearLogRecur(n / 2)
-    for (i in 0..<n.toInt()) {
+    for (i in 0..<n) {
         count++
     }
     return count
@@ -133,7 +133,7 @@ fun main() {
     val n = 8
     println("输入数据大小 n = $n")
 
-    var count: Int = constant(n)
+    var count = constant(n)
     println("常数阶的操作数量 = $count")
 
     count = linear(n)
@@ -144,7 +144,8 @@ fun main() {
     count = quadratic(n)
     println("平方阶的操作数量 = $count")
     val nums = IntArray(n)
-    for (i in 0..<n) nums[i] = n - i // [n,n-1,...,2,1]
+    for (i in 0..<n)
+        nums[i] = n - i // [n,n-1,...,2,1]
     count = bubbleSort(nums)
     println("平方阶（冒泡排序）的操作数量 = $count")
 

codes/kotlin/chapter_computational_complexity/worst_best_time_complexity.kt

@@ -13,10 +13,9 @@ fun randomNumbers(n: Int): Array<Int?> {
     for (i in 0..<n) {
         nums[i] = i + 1
     }
-    // 随机打乱数组元素
     val mutableList = nums.toMutableList()
+    // 随机打乱数组元素
     mutableList.shuffle()
-    // Integer[] -> int[]
     val res = arrayOfNulls<Int>(n)
     for (i in 0..<n) {
         res[i] = mutableList[i]
@@ -39,8 +38,8 @@ fun findOne(nums: Array<Int?>): Int {
 fun main() {
     for (i in 0..9) {
         val n = 100
-        val nums: Array<Int?> = randomNumbers(n)
-        val index: Int = findOne(nums)
+        val nums = randomNumbers(n)
+        val index = findOne(nums)
         println("\n数组 [ 1, 2, ..., n ] 被打乱后 = ${nums.contentToString()}")
         println("数字 1 的索引为 $index")
     }