Add kotlin code for chapter_stack_and_queue and chapter_tree (#1197)

* 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

---------

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

docs/chapter_stack_and_queue/deque.md

@@ -327,7 +327,29 @@
 === "Kotlin"
 
     ```kotlin title="deque.kt"
+    /* 初始化双向队列 */
+    val deque = LinkedList<Int>()
+    
+    /* 元素入队 */
+    deque.offerLast(2)  // 添加至队尾
+    deque.offerLast(5)
+    deque.offerLast(4)
+    deque.offerFirst(3) // 添加至队首
+    deque.offerFirst(1)
+    
+    /* 访问元素 */
+    val peekFirst = deque.peekFirst() // 队首元素
+    val peekLast = deque.peekLast()   // 队尾元素
     
+    /* 元素出队 */
+    val popFirst = deque.pollFirst() // 队首元素出队
+    val popLast = deque.pollLast()   // 队尾元素出队
+    
+    /* 获取双向队列的长度 */
+    val size = deque.size
+    
+    /* 判断双向队列是否为空 */
+    val isEmpty = deque.isEmpty()
     ```
 
 === "Zig"

docs/chapter_stack_and_queue/queue.md

@@ -305,7 +305,27 @@
 === "Kotlin"
 
     ```kotlin title="queue.kt"
+    /* 初始化队列 */
+    val queue = LinkedList<Int>()
+    
+    /* 元素入队 */
+    queue.offer(1)
+    queue.offer(3)
+    queue.offer(2)
+    queue.offer(5)
+    queue.offer(4)
+    
+    /* 访问队首元素 */
+    val peek = queue.peek()
     
+    /* 元素出队 */
+    val pop = queue.poll()
+    
+    /* 获取队列的长度 */
+    val size = queue.size
+    
+    /* 判断队列是否为空 */
+    val isEmpty = queue.isEmpty()
     ```
 
 === "Zig"

docs/chapter_stack_and_queue/stack.md

@@ -299,7 +299,27 @@
 === "Kotlin"
 
     ```kotlin title="stack.kt"
+    /* 初始化栈 */
+    val stack = Stack<Int>()
+    
+    /* 元素入栈 */
+    stack.push(1)
+    stack.push(3)
+    stack.push(2)
+    stack.push(5)
+    stack.push(4)
+    
+    /* 访问栈顶元素 */
+    val peek = stack.peek()
     
+    /* 元素出栈 */
+    val pop = stack.pop()
+    
+    /* 获取栈的长度 */
+    val size = stack.size
+    
+    /* 判断是否为空 */
+    val isEmpty = stack.isEmpty()
     ```
 
 === "Zig"

docs/chapter_tree/array_representation_of_tree.md

@@ -115,7 +115,9 @@
 === "Kotlin"
 
     ```kotlin title=""
-
+    /* 二叉树的数组表示 */
+    // 使用 null 来表示空位
+    val tree = mutableListOf( 1, 2, 3, 4, null, 6, 7, 8, 9, null, null, 12, null, null, 15 )
     ```
 
 === "Zig"

docs/chapter_tree/avl_tree.md

@@ -203,7 +203,12 @@ AVL 树既是二叉搜索树，也是平衡二叉树，同时满足这两类二
 === "Kotlin"
 
     ```kotlin title=""
-
+    /* AVL 树节点类 */
+    class TreeNode(val _val: Int) {  // 节点值
+        val height: Int = 0          // 节点高度
+        val left: TreeNode? = null   // 左子节点
+        val right: TreeNode? = null  // 右子节点
+    }
     ```
 
 === "Zig"

docs/chapter_tree/binary_tree.md

@@ -179,7 +179,11 @@
 === "Kotlin"
 
     ```kotlin title=""
-
+    /* 二叉树节点类 */
+    class TreeNode(val _val: Int) {  // 节点值
+        val left: TreeNode? = null   // 左子节点引用
+        val right: TreeNode? = null  // 右子节点引用
+    }
     ```
 
 === "Zig"
@@ -406,7 +410,17 @@
 === "Kotlin"
 
     ```kotlin title="binary_tree.kt"
-
+    // 初始化节点
+    val n1 = TreeNode(1)
+    val n2 = TreeNode(2)
+    val n3 = TreeNode(3)
+    val n4 = TreeNode(4)
+    val n5 = TreeNode(5)
+    // 构建节点之间的引用（指针）
+    n1.left = n2
+    n1.right = n3
+    n2.left = n4
+    n2.right = n5
     ```
 
 === "Zig"
@@ -557,7 +571,12 @@
 === "Kotlin"
 
     ```kotlin title="binary_tree.kt"
-
+    val P = TreeNode(0)
+    // 在 n1 -> n2 中间插入节点 P
+    n1.left = P
+    P.left = n2
+    // 删除节点 P
+    n1.left = n2
     ```
 
 === "Zig"