[mypy] Fix type annotations for linked_stack.py, evaluate_postfix_notations.py, stack.py in data structures (#4409)

* [mypy] Fix type annotations for linked_stack.py, next_greater_element.py, stack.py

* Reformatted files according to black

data_structures/stacks/evaluate_postfix_notations.py

@@ -1,3 +1,5 @@
+from typing import Any, List
+
 """
 The Reverse Polish Nation also known as Polish postfix notation
 or simply postfix notation.
@@ -21,7 +23,7 @@ def evaluate_postfix(postfix_notation: list) -> int:
         return 0
 
     operations = {"+", "-", "*", "/"}
-    stack = []
+    stack: List[Any] = []
 
     for token in postfix_notation:
         if token in operations:

data_structures/stacks/linked_stack.py

@@ -1,5 +1,5 @@
 """ A Stack using a linked list like structure """
-from typing import Any
+from typing import Any, Optional
 
 
 class Node:
@@ -42,7 +42,7 @@ class LinkedStack:
     """
 
     def __init__(self) -> None:
-        self.top = None
+        self.top: Optional[Node] = None
 
     def __iter__(self):
         node = self.top
@@ -134,6 +134,8 @@ class LinkedStack:
         """
         if self.is_empty():
             raise IndexError("peek from empty stack")
+
+        assert self.top is not None
         return self.top.data
 
     def clear(self) -> None:

data_structures/stacks/stack.py

@@ -1,3 +1,6 @@
+from typing import List
+
+
 class StackOverflowError(BaseException):
     pass
 
@@ -12,7 +15,7 @@ class Stack:
     """
 
     def __init__(self, limit: int = 10):
-        self.stack = []
+        self.stack: List[int] = []
         self.limit = limit
 
     def __bool__(self) -> bool: