follow PEP585 typing (#767)

Signed-off-by: Qingpeng Li <qingpeng9802@gmail.com>

docs/chapter_array_and_linkedlist/linked_list.md

@@ -23,7 +23,7 @@
         """链表节点类"""
         def __init__(self, val: int):
             self.val: int = val                  # 节点值
-            self.next: Optional[ListNode] = None # 指向下一节点的引用
+            self.next: ListNode | None = None # 指向下一节点的引用
     ```
 
 === "C++"
@@ -740,8 +740,8 @@
         """双向链表节点类"""
         def __init__(self, val: int):
             self.val: int = val                   # 节点值
-            self.next: Optional[ListNode] = None  # 指向后继节点的引用
-            self.prev: Optional[ListNode] = None  # 指向前驱节点的引用
+            self.next: ListNode | None = None  # 指向后继节点的引用
+            self.prev: ListNode | None = None  # 指向前驱节点的引用
     ```
 
 === "C++"

docs/chapter_computational_complexity/space_complexity.md

@@ -29,7 +29,7 @@
         """类"""
         def __init__(self, x: int):
             self.val: int = x                 # 节点值
-            self.next: Optional[Node] = None  # 指向下一节点的引用
+            self.next: Node | None = None  # 指向下一节点的引用
 
     def function() -> int:
         """函数"""

docs/chapter_tree/avl_tree.md

@@ -28,8 +28,8 @@ AVL 树既是二叉搜索树也是平衡二叉树，同时满足这两类二叉
         def __init__(self, val: int):
             self.val: int = val                    # 节点值
             self.height: int = 0                   # 节点高度
-            self.left: Optional[TreeNode] = None   # 左子节点引用
-            self.right: Optional[TreeNode] = None  # 右子节点引用
+            self.left: TreeNode | None = None   # 左子节点引用
+            self.right: TreeNode | None = None  # 右子节点引用
     ```
 
 === "C++"

docs/chapter_tree/binary_tree.md

@@ -9,8 +9,8 @@
         """二叉树节点类"""
         def __init__(self, val: int):
             self.val: int = val                   # 节点值
-            self.left: Optional[TreeNode] = None  # 左子节点引用
-            self.right: Optional[TreeNode] = None # 右子节点引用
+            self.left: TreeNode | None = None  # 左子节点引用
+            self.right: TreeNode | None = None # 右子节点引用
     ```
 
 === "C++"