Fix Python code.

codes/python/chapter_stack_and_queue/array_stack.py

@@ -10,35 +10,35 @@ class ArrayStack:
 
     def __init__(self):
         """构造方法"""
-        self.__stack: list[int] = []
+        self._stack: list[int] = []
 
     def size(self) -> int:
         """获取栈的长度"""
-        return len(self.__stack)
+        return len(self._stack)
 
     def is_empty(self) -> bool:
         """判断栈是否为空"""
-        return self.__stack == []
+        return self._stack == []
 
     def push(self, item: int):
         """入栈"""
-        self.__stack.append(item)
+        self._stack.append(item)
 
     def pop(self) -> int:
         """出栈"""
         if self.is_empty():
             raise IndexError("栈为空")
-        return self.__stack.pop()
+        return self._stack.pop()
 
     def peek(self) -> int:
         """访问栈顶元素"""
         if self.is_empty():
             raise IndexError("栈为空")
-        return self.__stack[-1]
+        return self._stack[-1]
 
     def to_list(self) -> list[int]:
         """返回列表用于打印"""
-        return self.__stack
+        return self._stack
 
 
 """Driver Code"""