diff --git a/codes/kotlin/chapter_sorting/bubble_sort.kt b/codes/kotlin/chapter_sorting/bubble_sort.kt
index 4fb482550..12608d160 100644
--- a/codes/kotlin/chapter_sorting/bubble_sort.kt
+++ b/codes/kotlin/chapter_sorting/bubble_sort.kt
@@ -14,7 +14,7 @@ fun bubbleSort(nums: IntArray) {
         for (j in 0..<i) {
             if (nums[j] > nums[j + 1]) {
                 // 交换 nums[j] 与 nums[j + 1]
-                nums[j] = nums[j+1].also { nums[j+1] = nums[j] }
+                nums[j] = nums[j + 1].also { nums[j + 1] = nums[j] }
             }
         }
     }
diff --git a/codes/kotlin/chapter_sorting/bucket_sort.kt b/codes/kotlin/chapter_sorting/bucket_sort.kt
index e4b1d9ff2..ccfcffc75 100644
--- a/codes/kotlin/chapter_sorting/bucket_sort.kt
+++ b/codes/kotlin/chapter_sorting/bucket_sort.kt
@@ -6,15 +6,13 @@
 
 package chapter_sorting
 
-import kotlin.collections.ArrayList
-
 /* 桶排序 */
 fun bucketSort(nums: FloatArray) {
     // 初始化 k = n/2 个桶，预期向每个桶分配 2 个元素
     val k = nums.size / 2
-    val buckets = ArrayList<ArrayList<Float>>()
+    val buckets = mutableListOf<MutableList<Float>>()
     for (i in 0..<k) {
-        buckets.add(ArrayList())
+        buckets.add(mutableListOf())
     }
     // 1. 将数组元素分配到各个桶中
     for (num in nums) {
diff --git a/codes/kotlin/chapter_sorting/counting_sort.kt b/codes/kotlin/chapter_sorting/counting_sort.kt
index 13e9b6df9..d08e8ce9f 100644
--- a/codes/kotlin/chapter_sorting/counting_sort.kt
+++ b/codes/kotlin/chapter_sorting/counting_sort.kt
@@ -14,7 +14,7 @@ fun countingSortNaive(nums: IntArray) {
     // 1. 统计数组最大元素 m
     var m = 0
     for (num in nums) {
-        m = max(m.toDouble(), num.toDouble()).toInt()
+        m = max(m, num)
     }
     // 2. 统计各数字的出现次数
     // counter[num] 代表 num 的出现次数
@@ -40,7 +40,7 @@ fun countingSort(nums: IntArray) {
     // 1. 统计数组最大元素 m
     var m = 0
     for (num in nums) {
-        m = max(m.toDouble(), num.toDouble()).toInt()
+        m = max(m, num)
     }
     // 2. 统计各数字的出现次数
     // counter[num] 代表 num 的出现次数
diff --git a/codes/kotlin/chapter_sorting/heap_sort.kt b/codes/kotlin/chapter_sorting/heap_sort.kt
index 9cb1f45d6..745d80ff1 100644
--- a/codes/kotlin/chapter_sorting/heap_sort.kt
+++ b/codes/kotlin/chapter_sorting/heap_sort.kt
@@ -14,10 +14,13 @@ fun siftDown(nums: IntArray, n: Int, li: Int) {
         val l = 2 * i + 1
         val r = 2 * i + 2
         var ma = i
-        if (l < n && nums[l] > nums[ma]) ma = l
-        if (r < n && nums[r] > nums[ma]) ma = r
+        if (l < n && nums[l] > nums[ma]) 
+            ma = l
+        if (r < n && nums[r] > nums[ma]) 
+            ma = r
         // 若节点 i 最大或索引 l, r 越界，则无须继续堆化，跳出
-        if (ma == i) break
+        if (ma == i) 
+            break
         // 交换两节点
         nums[i] = nums[ma].also { nums[ma] = nums[i] }
         // 循环向下堆化
@@ -45,4 +48,4 @@ fun main() {
     val nums = intArrayOf(4, 1, 3, 1, 5, 2)
     heapSort(nums)
     println("堆排序完成后 nums = ${nums.contentToString()}")
-}
\ No newline at end of file
+}
diff --git a/codes/kotlin/chapter_sorting/insertion_sort.kt b/codes/kotlin/chapter_sorting/insertion_sort.kt
index c16947307..015127d97 100644
--- a/codes/kotlin/chapter_sorting/insertion_sort.kt
+++ b/codes/kotlin/chapter_sorting/insertion_sort.kt
@@ -12,7 +12,7 @@ fun insertionSort(nums: IntArray) {
     for (i in nums.indices) {
         val base = nums[i]
         var j = i - 1
-        // 内循环: 将 base 插入到已排序部分的正确位置
+        // 内循环：将 base 插入到已排序区间 [0, i-1] 中的正确位置
         while (j >= 0 && nums[j] > base) {
             nums[j + 1] = nums[j] // 将 nums[j] 向右移动一位
             j--
@@ -26,4 +26,4 @@ fun main() {
     val nums = intArrayOf(4, 1, 3, 1, 5, 2)
     insertionSort(nums)
     println("插入排序完成后 nums = ${nums.contentToString()}")
-}
\ No newline at end of file
+}
diff --git a/codes/kotlin/chapter_sorting/merge_sort.kt b/codes/kotlin/chapter_sorting/merge_sort.kt
index 291ddde1f..113875422 100644
--- a/codes/kotlin/chapter_sorting/merge_sort.kt
+++ b/codes/kotlin/chapter_sorting/merge_sort.kt
@@ -17,8 +17,10 @@ fun merge(nums: IntArray, left: Int, mid: Int, right: Int) {
     var k = 0
     // 当左右子数组都还有元素时，进行比较并将较小的元素复制到临时数组中
     while (i <= mid && j <= right) {
-        if (nums[i] <= nums[j]) tmp[k++] = nums[i++]
-        else tmp[k++] = nums[j++]
+        if (nums[i] <= nums[j])
+            tmp[k++] = nums[i++]
+        else 
+            tmp[k++] = nums[j++]
     }
     // 将左子数组和右子数组的剩余元素复制到临时数组中
     while (i <= mid) {
@@ -51,4 +53,4 @@ fun main() {
     val nums = intArrayOf(7, 3, 2, 6, 0, 1, 5, 4)
     mergeSort(nums, 0, nums.size - 1)
     println("归并排序完成后 nums = ${nums.contentToString()}")
-}
\ No newline at end of file
+}
diff --git a/codes/kotlin/chapter_sorting/radix_sort.kt b/codes/kotlin/chapter_sorting/radix_sort.kt
index 16aa13c98..1fefdccf1 100644
--- a/codes/kotlin/chapter_sorting/radix_sort.kt
+++ b/codes/kotlin/chapter_sorting/radix_sort.kt
@@ -20,7 +20,7 @@ fun countingSortDigit(nums: IntArray, exp: Int) {
     // 统计 0~9 各数字的出现次数
     for (i in 0..<n) {
         val d = digit(nums[i], exp) // 获取 nums[i] 第 k 位，记为 d
-        counter[d]++ // 统计数字 d 的出现次数
+        counter[d]++                // 统计数字 d 的出现次数
     }
     // 求前缀和，将“出现个数”转换为“数组索引”
     for (i in 1..9) {
@@ -31,11 +31,12 @@ fun countingSortDigit(nums: IntArray, exp: Int) {
     for (i in n - 1 downTo 0) {
         val d = digit(nums[i], exp)
         val j = counter[d] - 1 // 获取 d 在数组中的索引 j
-        res[j] = nums[i] // 将当前元素填入索引 j
-        counter[d]-- // 将 d 的数量减 1
+        res[j] = nums[i]       // 将当前元素填入索引 j
+        counter[d]--           // 将 d 的数量减 1
     }
     // 使用结果覆盖原数组 nums
-    for (i in 0..<n) nums[i] = res[i]
+    for (i in 0..<n)
+        nums[i] = res[i]
 }
 
 /* 基数排序 */
diff --git a/codes/kotlin/chapter_sorting/selection_sort.kt b/codes/kotlin/chapter_sorting/selection_sort.kt
index 61eb173c3..823d4ff70 100644
--- a/codes/kotlin/chapter_sorting/selection_sort.kt
+++ b/codes/kotlin/chapter_sorting/selection_sort.kt
@@ -14,7 +14,8 @@ fun selectionSort(nums: IntArray) {
         var k = i
         // 内循环：找到未排序区间内的最小元素
         for (j in i + 1..<n) {
-            if (nums[j] < nums[k]) k = j // 记录最小元素的索引
+            if (nums[j] < nums[k])
+                k = j // 记录最小元素的索引
         }
         // 将该最小元素与未排序区间的首个元素交换
         nums[i] = nums[k].also { nums[k] = nums[i] }
diff --git a/codes/kotlin/chapter_stack_and_queue/array_deque.kt b/codes/kotlin/chapter_stack_and_queue/array_deque.kt
index d4997e003..605cc8a7c 100644
--- a/codes/kotlin/chapter_stack_and_queue/array_deque.kt
+++ b/codes/kotlin/chapter_stack_and_queue/array_deque.kt
@@ -7,10 +7,11 @@
 package chapter_stack_and_queue
 
 /* 基于环形数组实现的双向队列 */
+/* 构造方法 */
 class ArrayDeque(capacity: Int) {
-    private var nums = IntArray(capacity) // 用于存储双向队列元素的数组
-    private var front = 0 // 队首指针，指向队首元素
-    private var queSize = 0 // 双向队列长度
+    private var nums: IntArray = IntArray(capacity) // 用于存储双向队列元素的数组
+    private var front: Int = 0 // 队首指针，指向队首元素
+    private var queSize: Int = 0 // 双向队列长度
 
     /* 获取双向队列的容量 */
     fun capacity(): Int {
@@ -71,7 +72,7 @@ class ArrayDeque(capacity: Int) {
         return num
     }
 
-    /* 访问队尾元素 */
+    /* 队尾出队 */
     fun popLast(): Int {
         val num = peekLast()
         queSize--
diff --git a/codes/kotlin/chapter_stack_and_queue/array_queue.kt b/codes/kotlin/chapter_stack_and_queue/array_queue.kt
index eaf6a7cce..646767969 100644
--- a/codes/kotlin/chapter_stack_and_queue/array_queue.kt
+++ b/codes/kotlin/chapter_stack_and_queue/array_queue.kt
@@ -8,9 +8,9 @@ package chapter_stack_and_queue
 
 /* 基于环形数组实现的队列 */
 class ArrayQueue(capacity: Int) {
-    private val nums = IntArray(capacity) // 用于存储队列元素的数组
-    private var front = 0 // 队首指针，指向队首元素
-    private var queSize = 0 // 队列长度
+    private val nums: IntArray = IntArray(capacity) // 用于存储队列元素的数组
+    private var front: Int = 0 // 队首指针，指向队首元素
+    private var queSize: Int = 0 // 队列长度
 
     /* 获取队列的容量 */
     fun capacity(): Int {
diff --git a/codes/kotlin/chapter_stack_and_queue/array_stack.kt b/codes/kotlin/chapter_stack_and_queue/array_stack.kt
index efef9aab5..0aee3034f 100644
--- a/codes/kotlin/chapter_stack_and_queue/array_stack.kt
+++ b/codes/kotlin/chapter_stack_and_queue/array_stack.kt
@@ -9,7 +9,7 @@ package chapter_stack_and_queue
 /* 基于数组实现的栈 */
 class ArrayStack {
     // 初始化列表（动态数组）
-    private val stack = ArrayList<Int>()
+    private val stack = mutableListOf<Int>()
 
     /* 获取栈的长度 */
     fun size(): Int {
@@ -40,7 +40,7 @@ class ArrayStack {
 
     /* 将 List 转化为 Array 并返回 */
     fun toArray(): Array<Any> {
-        return stack.toArray()
+        return stack.toTypedArray()
     }
 }
 
@@ -63,7 +63,7 @@ fun main() {
 
     /* 元素出栈 */
     val pop = stack.pop()
-    println("出栈元素 pop = ${pop}，出栈后 stack = ${stack.toArray().contentToString()}")
+    println("出栈元素 pop = $pop，出栈后 stack = ${stack.toArray().contentToString()}")
 
     /* 获取栈的长度 */
     val size = stack.size()
diff --git a/codes/kotlin/chapter_stack_and_queue/linkedlist_deque.kt b/codes/kotlin/chapter_stack_and_queue/linkedlist_deque.kt
index cc2a5c232..0f2789957 100644
--- a/codes/kotlin/chapter_stack_and_queue/linkedlist_deque.kt
+++ b/codes/kotlin/chapter_stack_and_queue/linkedlist_deque.kt
@@ -7,7 +7,7 @@
 package chapter_stack_and_queue
 
 /* 双向链表节点 */
-class ListNode(var value: Int) {
+class ListNode(var _val: Int) {
     // 节点值
     var next: ListNode? = null // 后继节点引用
     var prev: ListNode? = null // 前驱节点引用
@@ -15,9 +15,9 @@ class ListNode(var value: Int) {
 
 /* 基于双向链表实现的双向队列 */
 class LinkedListDeque {
-    private var front: ListNode? = null // 头节点 front ，尾节点 rear
-    private var rear: ListNode? = null
-    private var queSize = 0 // 双向队列的长度
+    private var front: ListNode? = null // 头节点 front
+    private var rear: ListNode? = null // 尾节点 rear
+    private var queSize: Int = 0 // 双向队列的长度
 
     /* 获取双向队列的长度 */
     fun size(): Int {
@@ -64,12 +64,12 @@ class LinkedListDeque {
 
     /* 出队操作 */
     fun pop(isFront: Boolean): Int {
-        if (isEmpty()) throw IndexOutOfBoundsException()
-
+        if (isEmpty()) 
+            throw IndexOutOfBoundsException()
         val value: Int
         // 队首出队操作
         if (isFront) {
-            value = front!!.value // 暂存头节点值
+            value = front!!._val // 暂存头节点值
             // 删除头节点
             val fNext = front!!.next
             if (fNext != null) {
@@ -79,7 +79,7 @@ class LinkedListDeque {
             front = fNext // 更新头节点
             // 队尾出队操作
         } else {
-            value = rear!!.value // 暂存尾节点值
+            value = rear!!._val // 暂存尾节点值
             // 删除尾节点
             val rPrev = rear!!.prev
             if (rPrev != null) {
@@ -104,17 +104,14 @@ class LinkedListDeque {
 
     /* 访问队首元素 */
     fun peekFirst(): Int {
-        if (isEmpty()) {
-            throw IndexOutOfBoundsException()
-
-        }
-        return front!!.value
+        if (isEmpty()) throw IndexOutOfBoundsException()
+        return front!!._val
     }
 
     /* 访问队尾元素 */
     fun peekLast(): Int {
         if (isEmpty()) throw IndexOutOfBoundsException()
-        return rear!!.value
+        return rear!!._val
     }
 
     /* 返回数组用于打印 */
@@ -122,7 +119,7 @@ class LinkedListDeque {
         var node = front
         val res = IntArray(size())
         for (i in res.indices) {
-            res[i] = node!!.value
+            res[i] = node!!._val
             node = node.next
         }
         return res
@@ -163,4 +160,4 @@ fun main() {
     /* 判断双向队列是否为空 */
     val isEmpty = deque.isEmpty()
     println("双向队列是否为空 = $isEmpty")
-}
\ No newline at end of file
+}
diff --git a/codes/kotlin/chapter_stack_and_queue/linkedlist_queue.kt b/codes/kotlin/chapter_stack_and_queue/linkedlist_queue.kt
index 33f48d560..64720de8a 100644
--- a/codes/kotlin/chapter_stack_and_queue/linkedlist_queue.kt
+++ b/codes/kotlin/chapter_stack_and_queue/linkedlist_queue.kt
@@ -52,7 +52,7 @@ class LinkedListQueue(
     /* 访问队首元素 */
     fun peek(): Int {
         if (isEmpty()) throw IndexOutOfBoundsException()
-        return front!!.value
+        return front!!._val
     }
 
     /* 将链表转化为 Array 并返回 */
@@ -60,7 +60,7 @@ class LinkedListQueue(
         var node = front
         val res = IntArray(size())
         for (i in res.indices) {
-            res[i] = node!!.value
+            res[i] = node!!._val
             node = node.next
         }
         return res
diff --git a/codes/kotlin/chapter_stack_and_queue/linkedlist_stack.kt b/codes/kotlin/chapter_stack_and_queue/linkedlist_stack.kt
index f51b0b9eb..b63f3639a 100644
--- a/codes/kotlin/chapter_stack_and_queue/linkedlist_stack.kt
+++ b/codes/kotlin/chapter_stack_and_queue/linkedlist_stack.kt
@@ -41,7 +41,7 @@ class LinkedListStack(
     /* 访问栈顶元素 */
     fun peek(): Int? {
         if (isEmpty()) throw IndexOutOfBoundsException()
-        return stackPeek?.value
+        return stackPeek?._val
     }
 
     /* 将 List 转化为 Array 并返回 */
@@ -49,7 +49,7 @@ class LinkedListStack(
         var node = stackPeek
         val res = IntArray(size())
         for (i in res.size - 1 downTo 0) {
-            res[i] = node?.value!!
+            res[i] = node?._val!!
             node = node.next
         }
         return res
diff --git a/codes/kotlin/chapter_tree/array_binary_tree.kt b/codes/kotlin/chapter_tree/array_binary_tree.kt
index 46c09cef0..f40c30758 100644
--- a/codes/kotlin/chapter_tree/array_binary_tree.kt
+++ b/codes/kotlin/chapter_tree/array_binary_tree.kt
@@ -10,7 +10,8 @@ import utils.TreeNode
 import utils.printTree
 
 /* 数组表示下的二叉树类 */
-class ArrayBinaryTree(private val tree: List<Int?>) {
+/* 构造方法 */
+class ArrayBinaryTree(private val tree: MutableList<Int?>) {
     /* 列表容量 */
     fun size(): Int {
         return tree.size
@@ -39,11 +40,12 @@ class ArrayBinaryTree(private val tree: List<Int?>) {
     }
 
     /* 层序遍历 */
-    fun levelOrder(): List<Int?> {
-        val res = ArrayList<Int?>()
+    fun levelOrder(): MutableList<Int?> {
+        val res = mutableListOf<Int?>()
         // 直接遍历数组
         for (i in 0..<size()) {
-            if (value(i) != null) res.add(value(i))
+            if (value(i) != null)
+                res.add(value(i))
         }
         return res
     }
@@ -51,34 +53,38 @@ class ArrayBinaryTree(private val tree: List<Int?>) {
     /* 深度优先遍历 */
     fun dfs(i: Int, order: String, res: MutableList<Int?>) {
         // 若为空位，则返回
-        if (value(i) == null) return
+        if (value(i) == null)
+            return
         // 前序遍历
-        if ("pre" == order) res.add(value(i))
+        if ("pre" == order)
+            res.add(value(i))
         dfs(left(i), order, res)
         // 中序遍历
-        if ("in" == order) res.add(value(i))
+        if ("in" == order)
+            res.add(value(i))
         dfs(right(i), order, res)
         // 后序遍历
-        if ("post" == order) res.add(value(i))
+        if ("post" == order)
+            res.add(value(i))
     }
 
     /* 前序遍历 */
-    fun preOrder(): List<Int?> {
-        val res = ArrayList<Int?>()
+    fun preOrder(): MutableList<Int?> {
+        val res = mutableListOf<Int?>()
         dfs(0, "pre", res)
         return res
     }
 
     /* 中序遍历 */
-    fun inOrder(): List<Int?> {
-        val res = ArrayList<Int?>()
+    fun inOrder(): MutableList<Int?> {
+        val res = mutableListOf<Int?>()
         dfs(0, "in", res)
         return res
     }
 
     /* 后序遍历 */
-    fun postOrder(): List<Int?> {
-        val res = ArrayList<Int?>()
+    fun postOrder(): MutableList<Int?> {
+        val res = mutableListOf<Int?>()
         dfs(0, "post", res)
         return res
     }
diff --git a/codes/kotlin/chapter_tree/avl_tree.kt b/codes/kotlin/chapter_tree/avl_tree.kt
index 8f2d18b24..634b75081 100644
--- a/codes/kotlin/chapter_tree/avl_tree.kt
+++ b/codes/kotlin/chapter_tree/avl_tree.kt
@@ -23,7 +23,7 @@ class AVLTree {
     /* 更新节点高度 */
     private fun updateHeight(node: TreeNode?) {
         // 节点高度等于最高子树高度 + 1
-        node?.height = (max(height(node?.left).toDouble(), height(node?.right).toDouble()) + 1).toInt()
+        node?.height = max(height(node?.left), height(node?.right)) + 1
     }
 
     /* 获取平衡因子 */
@@ -103,10 +103,12 @@ class AVLTree {
             return TreeNode(value)
         var node = n
         /* 1. 查找插入位置并插入节点 */
-        if (value < node.value) node.left = insertHelper(node.left, value)
-        else if (value > node.value) node.right = insertHelper(node.right, value)
-        else return node // 重复节点不插入，直接返回
-
+        if (value < node.value)
+            node.left = insertHelper(node.left, value)
+        else if (value > node.value)
+            node.right = insertHelper(node.right, value)
+        else
+            return node // 重复节点不插入，直接返回
         updateHeight(node) // 更新节点高度
         /* 2. 执行旋转操作，使该子树重新恢复平衡 */
         node = rotate(node)
@@ -123,14 +125,22 @@ class AVLTree {
     private fun removeHelper(n: TreeNode?, value: Int): TreeNode? {
         var node = n ?: return null
         /* 1. 查找节点并删除 */
-        if (value < node.value) node.left = removeHelper(node.left, value)
-        else if (value > node.value) node.right = removeHelper(node.right, value)
+        if (value < node.value)
+            node.left = removeHelper(node.left, value)
+        else if (value > node.value)
+            node.right = removeHelper(node.right, value)
         else {
             if (node.left == null || node.right == null) {
-                val child = if (node.left != null) node.left else node.right
+                val child = if (node.left != null)
+                    node.left
+                else
+                    node.right
                 // 子节点数量 = 0 ，直接删除 node 并返回
-                if (child == null) return null
-                else node = child
+                if (child == null)
+                    return null
+                // 子节点数量 = 1 ，直接删除 node
+                else
+                    node = child
             } else {
                 // 子节点数量 = 2 ，则将中序遍历的下个节点删除，并用该节点替换当前节点
                 var temp = node.right
@@ -154,9 +164,14 @@ class AVLTree {
         // 循环查找，越过叶节点后跳出
         while (cur != null) {
             // 目标节点在 cur 的右子树中
-            cur = if (cur.value < value) cur.right!!
-            else (if (cur.value > value) cur.left
-            else break)!!
+            cur = if (cur.value < value)
+                cur.right!!
+            // 目标节点在 cur 的左子树中
+            else if (cur.value > value)
+                cur.left
+            // 找到目标节点，跳出循环
+            else
+                break
         }
         // 返回目标节点
         return cur
diff --git a/codes/kotlin/chapter_tree/binary_search_tree.kt b/codes/kotlin/chapter_tree/binary_search_tree.kt
index 0d4513e0c..5dd01fe69 100644
--- a/codes/kotlin/chapter_tree/binary_search_tree.kt
+++ b/codes/kotlin/chapter_tree/binary_search_tree.kt
@@ -11,6 +11,7 @@ import utils.printTree
 
 /* 二叉搜索树 */
 class BinarySearchTree {
+    // 初始化空树
     private var root: TreeNode? = null
 
     /* 获取二叉树根节点 */
@@ -24,11 +25,14 @@ class BinarySearchTree {
         // 循环查找，越过叶节点后跳出
         while (cur != null) {
             // 目标节点在 cur 的右子树中
-            cur = if (cur.value < num) cur.right
+            cur = if (cur.value < num)
+                cur.right
             // 目标节点在 cur 的左子树中
-            else if (cur.value > num) cur.left
+            else if (cur.value > num)
+                cur.left
             // 找到目标节点，跳出循环
-            else break
+            else
+                break
         }
         // 返回目标节点
         return cur
@@ -46,45 +50,60 @@ class BinarySearchTree {
         // 循环查找，越过叶节点后跳出
         while (cur != null) {
             // 找到重复节点，直接返回
-            if (cur.value == num) return
+            if (cur.value == num)
+                return
             pre = cur
             // 插入位置在 cur 的右子树中
-            cur = if (cur.value < num) cur.right
+            cur = if (cur.value < num)
+                cur.right
             // 插入位置在 cur 的左子树中
-            else cur.left
+            else
+                cur.left
         }
         // 插入节点
         val node = TreeNode(num)
-        if (pre?.value!! < num) pre.right = node
-        else pre.left = node
+        if (pre?.value!! < num)
+            pre.right = node
+        else
+            pre.left = node
     }
 
     /* 删除节点 */
     fun remove(num: Int) {
         // 若树为空，直接提前返回
-        if (root == null) return
+        if (root == null)
+            return
         var cur = root
         var pre: TreeNode? = null
         // 循环查找，越过叶节点后跳出
         while (cur != null) {
             // 找到待删除节点，跳出循环
-            if (cur.value == num) break
+            if (cur.value == num)
+                break
             pre = cur
             // 待删除节点在 cur 的右子树中
-            cur = if (cur.value < num) cur.right
+            cur = if (cur.value < num)
+                cur.right
             // 待删除节点在 cur 的左子树中
-            else cur.left
+            else
+                cur.left
         }
         // 若无待删除节点，则直接返回
-        if (cur == null) return
+        if (cur == null)
+            return
         // 子节点数量 = 0 or 1
         if (cur.left == null || cur.right == null) {
             // 当子节点数量 = 0 / 1 时， child = null / 该子节点
-            val child = if (cur.left != null) cur.left else cur.right
+            val child = if (cur.left != null)
+                cur.left
+            else
+                cur.right
             // 删除节点 cur
             if (cur != root) {
-                if (pre!!.left == cur) pre.left = child
-                else pre.right = child
+                if (pre!!.left == cur)
+                    pre.left = child
+                else
+                    pre.right = child
             } else {
                 // 若删除节点为根节点，则重新指定根节点
                 root = child
diff --git a/codes/kotlin/chapter_tree/binary_tree_bfs.kt b/codes/kotlin/chapter_tree/binary_tree_bfs.kt
index 3bd6b850d..9316e5186 100644
--- a/codes/kotlin/chapter_tree/binary_tree_bfs.kt
+++ b/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")
-}
\ No newline at end of file
+}
diff --git a/codes/kotlin/chapter_tree/binary_tree_dfs.kt b/codes/kotlin/chapter_tree/binary_tree_dfs.kt
index cefae4e27..aa879c435 100644
--- a/codes/kotlin/chapter_tree/binary_tree_dfs.kt
+++ b/codes/kotlin/chapter_tree/binary_tree_dfs.kt
@@ -10,7 +10,7 @@ import utils.TreeNode
 import utils.printTree
 
 // 初始化列表，用于存储遍历序列
-var list = ArrayList<Int>()
+var list = mutableListOf<Int>()
 
 /* 前序遍历 */
 fun preOrder(root: TreeNode?) {
diff --git a/codes/kotlin/utils/PrintUtil.kt b/codes/kotlin/utils/PrintUtil.kt
index f81984e27..9954a4621 100644
--- a/codes/kotlin/utils/PrintUtil.kt
+++ b/codes/kotlin/utils/PrintUtil.kt
@@ -20,7 +20,7 @@ fun <T> printMatrix(matrix: Array<Array<T>>) {
 }
 
 /* 打印矩阵（List） */
-fun <T> printMatrix(matrix: List<List<T>>) {
+fun <T> printMatrix(matrix: MutableList<MutableList<T>>) {
     println("[")
     for (row in matrix) {
         println("  $row,")
@@ -31,7 +31,7 @@ fun <T> printMatrix(matrix: List<List<T>>) {
 /* 打印链表 */
 fun printLinkedList(h: ListNode?) {
     var head = h
-    val list = ArrayList<String>()
+    val list = mutableListOf<String>()
     while (head != null) {
         list.add(head.value.toString())
         head = head.next
@@ -91,16 +91,17 @@ fun showTrunks(p: Trunk?) {
 /* 打印哈希表 */
 fun <K, V> printHashMap(map: Map<K, V>) {
     for ((key, value) in map) {
-        println(key.toString() + " -> " + value)
+        println("${key.toString()} -> $value")
     }
 }
 
 /* 打印堆 */
 fun printHeap(queue: Queue<Int>?) {
-    val list = queue?.let { ArrayList(it) }
+    val list = mutableListOf<Int?>()
+    queue?.let { list.addAll(it) }
     print("堆的数组表示：")
     println(list)
     println("堆的树状表示：")
-    val root = list?.let { TreeNode.listToTree(it) }
+    val root = TreeNode.listToTree(list)
     printTree(root)
 }
diff --git a/codes/kotlin/utils/TreeNode.kt b/codes/kotlin/utils/TreeNode.kt
index a9b6e1777..e3a21c241 100644
--- a/codes/kotlin/utils/TreeNode.kt
+++ b/codes/kotlin/utils/TreeNode.kt
@@ -7,6 +7,7 @@
 package utils
 
 /* 二叉树节点类 */
+/* 构造方法 */
 class TreeNode(
     var value: Int // 节点值
 ) {
@@ -59,8 +60,8 @@ class TreeNode(
         }
 
         /* 将二叉树序列化为列表 */
-        fun treeToList(root: TreeNode?): List<Int?> {
-            val res = ArrayList<Int?>()
+        fun treeToList(root: TreeNode?): MutableList<Int?> {
+            val res = mutableListOf<Int?>()
             treeToListDFS(root, 0, res)
             return res
         }
diff --git a/codes/kotlin/utils/Vertex.kt b/codes/kotlin/utils/Vertex.kt
index 2abe51fb2..fb1f9de9e 100644
--- a/codes/kotlin/utils/Vertex.kt
+++ b/codes/kotlin/utils/Vertex.kt
@@ -19,8 +19,8 @@ class Vertex(val value: Int) {
         }
 
         /* 输入顶点列表 vets ，返回值列表 vals */
-        fun vetsToVals(vets: List<Vertex?>): List<Int> {
-            val vals = ArrayList<Int>()
+        fun vetsToVals(vets: MutableList<Vertex?>): MutableList<Int> {
+            val vals = mutableListOf<Int>()
             for (vet in vets) {
                 vals.add(vet!!.value)
             }