[mypy] Add/fix type annotations for stacks in data structures

data_structures/stacks/evaluate_postfix_notations.py

@@ -7,7 +7,7 @@
 Each operand may be an integer or another expression.
 """
 
-
+from typing import List
 def evaluate_postfix(postfix_notation: list) -> int:
     """
     >>> evaluate_postfix(["2", "1", "+", "3", "*"])
@@ -21,8 +21,9 @@ def evaluate_postfix(postfix_notation: list) -> int:
         return 0
 
     operations = {"+", "-", "*", "/"}
-    stack = []
 
+    stack: List[int]
+
     for token in postfix_notation:
         if token in operations:
             b, a = stack.pop(), stack.pop()

data_structures/stacks/next_greater_element.py

@@ -1,3 +1,4 @@
+from typing import List
 arr = [-10, -5, 0, 5, 5.1, 11, 13, 21, 3, 4, -21, -10, -5, -1, 0]
 expect = [-5, 0, 5, 5.1, 11, 13, 21, -1, 4, -1, -10, -5, -1, 0, -1]
 
@@ -53,7 +54,7 @@ def next_greatest_element(arr: list) -> list:
     >>> next_greatest_element(arr) == expect
     True
     """
-    stack = []
+    stack: List[int]
     result = [-1] * len(arr)
 
     for index in reversed(range(len(arr))):

data_structures/stacks/stack.py

@@ -1,3 +1,4 @@
+from typing import List
 class StackOverflowError(BaseException):
     pass
 
@@ -10,9 +11,9 @@ class Stack:
     Last In, First Out (LIFO).
     https://en.wikipedia.org/wiki/Stack_(abstract_data_type)
     """
-
+    
     def __init__(self, limit: int = 10):
-        self.stack = []
+        self.stack: List[int]
         self.limit = limit
 
     def __bool__(self) -> bool:
@@ -22,28 +23,28 @@ def __str__(self) -> str:
         return str(self.stack)
 
     def push(self, data):
-        """Push an element to the top of the stack."""
+        """ Push an element to the top of the stack."""
         if len(self.stack) >= self.limit:
             raise StackOverflowError
         self.stack.append(data)
 
     def pop(self):
-        """Pop an element off of the top of the stack."""
+        """ Pop an element off of the top of the stack."""
         return self.stack.pop()
 
     def peek(self):
-        """Peek at the top-most element of the stack."""
+        """ Peek at the top-most element of the stack."""
         return self.stack[-1]
 
     def is_empty(self) -> bool:
-        """Check if a stack is empty."""
+        """ Check if a stack is empty."""
         return not bool(self.stack)
 
     def is_full(self) -> bool:
         return self.size() == self.limit
 
     def size(self) -> int:
-        """Return the size of the stack."""
+        """ Return the size of the stack."""
         return len(self.stack)
 
     def __contains__(self, item) -> bool: