Added postfix-prefix conversion

DIRECTORY.md

@@ -257,6 +257,7 @@
     * [Dijkstras Two Stack Algorithm](data_structures/stacks/dijkstras_two_stack_algorithm.py)
     * [Infix To Postfix Conversion](data_structures/stacks/infix_to_postfix_conversion.py)
     * [Infix To Prefix Conversion](data_structures/stacks/infix_to_prefix_conversion.py)
+    * [Postfix to Prefix Conversion](data_structures/stacks/postfix_to_prefix_conversion.py)
     * [Next Greater Element](data_structures/stacks/next_greater_element.py)
     * [Postfix Evaluation](data_structures/stacks/postfix_evaluation.py)
     * [Prefix Evaluation](data_structures/stacks/prefix_evaluation.py)

data_structures/stacks/postfix_to_prefix_conversion.py

@@ -0,0 +1,49 @@
+"""
+Functions:
+- postfix_to_prefix(expression): Converts a postfix expression to a prefix expression.
+"""
+
+from .stack import Stack
+
+
+def postfix_to_prefix(expression_str: str) -> str:
+    """
+    Converts a postfix expression to a prefix expression.
+
+    Args:
+        expression (str): A string representing a postfix expression.
+
+    Returns:
+        str: A string representing the equivalent prefix expression.
+
+    Raises:
+        IndexError: If the expression is invalid or contains an invalid operator.
+
+    Example:
+        >>> postfix_to_prefix('23+5*')
+        '*+235'
+    """
+    stack: Stack[str] = Stack()
+    operators = {"+", "-", "*", "/", "^"}
+
+    for char in expression_str:
+        if char not in operators:
+            stack.push(char)
+        else:
+            operand1 = stack.pop()
+            operand2 = stack.pop()
+            stack.push(char + operand2 + operand1)
+
+    return stack.pop()
+
+
+if __name__ == "__main__":
+    from doctest import testmod
+
+    testmod()
+    postfix_expression = "ab+c*"
+
+    print("Postfix to infix demo\n")
+    print("Postfix expression:", postfix_expression)
+    prefix_expression = postfix_to_prefix(postfix_expression)
+    print("Prefix expression:", prefix_expression)