diff --git a/DIRECTORY.md b/DIRECTORY.md
index ea5e01addeb01c61b2008d94ad9ea2edf38a95eb..7c695892112a68e931014dc5fa9ae7016d6c92a2 100644
--- a/DIRECTORY.md
+++ b/DIRECTORY.md
@@ -674,6 +674,8 @@
     * [Sol1](https://github.com/TheAlgorithms/Python/blob/master/project_euler/problem_056/sol1.py)
   * Problem 057
     * [Sol1](https://github.com/TheAlgorithms/Python/blob/master/project_euler/problem_057/sol1.py)
+  * Problem 058
+    * [Sol1](https://github.com/TheAlgorithms/Python/blob/master/project_euler/problem_058/sol1.py)
   * Problem 062
     * [Sol1](https://github.com/TheAlgorithms/Python/blob/master/project_euler/problem_062/sol1.py)
   * Problem 063
@@ -699,6 +701,8 @@
     * [Sol1](https://github.com/TheAlgorithms/Python/blob/master/project_euler/problem_080/sol1.py)
   * Problem 081
     * [Sol1](https://github.com/TheAlgorithms/Python/blob/master/project_euler/problem_081/sol1.py)
+  * Problem 087
+    * [Sol1](https://github.com/TheAlgorithms/Python/blob/master/project_euler/problem_087/sol1.py)
   * Problem 091
     * [Sol1](https://github.com/TheAlgorithms/Python/blob/master/project_euler/problem_091/sol1.py)
   * Problem 097
@@ -817,6 +821,7 @@
   * [Prefix Function](https://github.com/TheAlgorithms/Python/blob/master/strings/prefix_function.py)
   * [Rabin Karp](https://github.com/TheAlgorithms/Python/blob/master/strings/rabin_karp.py)
   * [Remove Duplicate](https://github.com/TheAlgorithms/Python/blob/master/strings/remove_duplicate.py)
+  * [Reverse Letters](https://github.com/TheAlgorithms/Python/blob/master/strings/reverse_letters.py)
   * [Reverse Words](https://github.com/TheAlgorithms/Python/blob/master/strings/reverse_words.py)
   * [Split](https://github.com/TheAlgorithms/Python/blob/master/strings/split.py)
   * [Swap Case](https://github.com/TheAlgorithms/Python/blob/master/strings/swap_case.py)
diff --git a/data_structures/stacks/__init__.py b/data_structures/stacks/__init__.py
index f6995cf989776523b66d2908c6dffcf9d9586b3d..e69de29bb2d1d6434b8b29ae775ad8c2e48c5391 100644
--- a/data_structures/stacks/__init__.py
+++ b/data_structures/stacks/__init__.py
@@ -1,22 +0,0 @@
-class Stack:
-    def __init__(self):
-        self.stack = []
-        self.top = 0
-
-    def is_empty(self):
-        return self.top == 0
-
-    def push(self, item):
-        if self.top < len(self.stack):
-            self.stack[self.top] = item
-        else:
-            self.stack.append(item)
-
-        self.top += 1
-
-    def pop(self):
-        if self.is_empty():
-            return None
-        else:
-            self.top -= 1
-            return self.stack[self.top]
diff --git a/data_structures/stacks/infix_to_postfix_conversion.py b/data_structures/stacks/infix_to_postfix_conversion.py
index 4a1180c9d8e45a61a939851401ad7d5f0e1daf16..dedba8479ac8a664e2a125bc01f70cfc4b41c699 100644
--- a/data_structures/stacks/infix_to_postfix_conversion.py
+++ b/data_structures/stacks/infix_to_postfix_conversion.py
@@ -1,57 +1,67 @@
-import string
+"""
+https://en.wikipedia.org/wiki/Infix_notation
+https://en.wikipedia.org/wiki/Reverse_Polish_notation
+https://en.wikipedia.org/wiki/Shunting-yard_algorithm
+"""
 
+from .balanced_parentheses import balanced_parentheses
 from .stack import Stack
 
-__author__ = "Omkar Pathak"
 
-
-def is_operand(char):
-    return char in string.ascii_letters or char in string.digits
-
-
-def precedence(char):
-    """Return integer value representing an operator's precedence, or
+def precedence(char: str) -> int:
+    """
+    Return integer value representing an operator's precedence, or
     order of operation.
-
     https://en.wikipedia.org/wiki/Order_of_operations
     """
-    dictionary = {"+": 1, "-": 1, "*": 2, "/": 2, "^": 3}
-    return dictionary.get(char, -1)
-
+    return {"+": 1, "-": 1, "*": 2, "/": 2, "^": 3}.get(char, -1)
 
-def infix_to_postfix(expression):
-    """Convert infix notation to postfix notation using the Shunting-yard
-    algorithm.
 
-    https://en.wikipedia.org/wiki/Shunting-yard_algorithm
-    https://en.wikipedia.org/wiki/Infix_notation
-    https://en.wikipedia.org/wiki/Reverse_Polish_notation
+def infix_to_postfix(expression_str: str) -> str:
+    """
+    >>> infix_to_postfix("(1*(2+3)+4))")
+    Traceback (most recent call last):
+    ...
+    ValueError: Mismatched parentheses
+    >>> infix_to_postfix("")
+    ''
+    >>> infix_to_postfix("3+2")
+    '3 2 +'
+    >>> infix_to_postfix("(3+4)*5-6")
+    '3 4 + 5 * 6 -'
+    >>> infix_to_postfix("(1+2)*3/4-5")
+    '1 2 + 3 * 4 / 5 -'
+    >>> infix_to_postfix("a+b*c+(d*e+f)*g")
+    'a b c * + d e * f + g * +'
+    >>> infix_to_postfix("x^y/(5*z)+2")
+    'x y ^ 5 z * / 2 +'
     """
-    stack = Stack(len(expression))
+    if not balanced_parentheses(expression_str):
+        raise ValueError("Mismatched parentheses")
+    stack = Stack()
     postfix = []
-    for char in expression:
-        if is_operand(char):
+    for char in expression_str:
+        if char.isalpha() or char.isdigit():
             postfix.append(char)
-        elif char not in {"(", ")"}:
-            while not stack.is_empty() and precedence(char) <= precedence(stack.peek()):
-                postfix.append(stack.pop())
-            stack.push(char)
         elif char == "(":
             stack.push(char)
         elif char == ")":
             while not stack.is_empty() and stack.peek() != "(":
                 postfix.append(stack.pop())
-            # Pop '(' from stack. If there is no '(', there is a mismatched
-            # parentheses.
-            if stack.peek() != "(":
-                raise ValueError("Mismatched parentheses")
             stack.pop()
+        else:
+            while not stack.is_empty() and precedence(char) <= precedence(stack.peek()):
+                postfix.append(stack.pop())
+            stack.push(char)
     while not stack.is_empty():
         postfix.append(stack.pop())
     return " ".join(postfix)
 
 
 if __name__ == "__main__":
+    from doctest import testmod
+
+    testmod()
     expression = "a+b*(c^d-e)^(f+g*h)-i"
 
     print("Infix to Postfix Notation demonstration:\n")