fix: raise error in peek() function of linkedlist_stack.py, linkedlist_queue.py, linkedlist_deque.py (#754)

* fix: linkedlist_stack.py 中的peek方法需要判空raise Error

栈空的时候不能再 peek() 。另外由于 pop() 复用了 peek() ，栈空时返回的 None 会传递到 pop() 中的 num ，导致后续的继续执行 
 self.peek.next 报错等后续问题

* Update linkedlist_stack.py

* Update linkedlist_queue.py

* Update linkedlist_deque.py

---------

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

codes/python/chapter_stack_and_queue/linkedlist_deque.py

@@ -96,11 +96,15 @@ class LinkedListDeque:
 
     def peek_first(self) -> int:
         """访问队首元素"""
-        return None if self.is_empty() else self.front.val
+        if self.is_empty():
+            raise IndexError("双向队列为空")
+        return self.front.val
 
     def peek_last(self) -> int:
         """访问队尾元素"""
-        return None if self.is_empty() else self.rear.val
+        if self.is_empty():
+            raise IndexError("双向队列为空")
+        return self.rear.val
 
     def to_array(self) -> list[int]:
         """返回数组用于打印"""

codes/python/chapter_stack_and_queue/linkedlist_queue.py

@@ -51,9 +51,8 @@ class LinkedListQueue:
 
     def peek(self) -> int:
         """访问队首元素"""
-        if self.size() == 0:
+        if self.is_empty():
-            print("队列为空")
+            raise IndexError("队列为空")
-            return False
         return self.__front.val
 
     def to_list(self) -> list[int]:

codes/python/chapter_stack_and_queue/linkedlist_stack.py

@@ -35,16 +35,15 @@ class LinkedListStack:
 
     def pop(self) -> int:
         """出栈"""
-        num: int = self.peek()
+        num = self.peek()
         self.__peek = self.__peek.next
         self.__size -= 1
         return num
 
     def peek(self) -> int:
         """访问栈顶元素"""
-        # 判空处理
+        if self.is_empty():
-        if not self.__peek:
+            raise IndexError("栈为空")
-            return None
         return self.__peek.val
 
     def to_list(self) -> list[int]: