Adding in the evaluate postfix notation using Stack

data_structures/stacks/evaluate_postfix_notations.py

@@ -0,0 +1,48 @@
+# The Reverse Polish Nation also known as Polish postfix notation or simply postfix notation
+# https://en.wikipedia.org/wiki/Reverse_Polish_notation
+# Classic examples of simple stack implementations
+# Valid operators are +, -, *, /. Each operand may be an integer or another expression.
+
+
+def evaluate_postfix(postfix_notation):
+    """
+    >>> array = ["10", "6", "9", "3", "+", "-11", "*", "/", "*", "17", "+", "5", "+"]
+    >>> evaluate_postfix(array)
+    22
+    >>> array = ["2", "1", "+", "3", "*"]
+    >>> evaluate_postfix(array)
+    9
+    >>> array = ["4", "13", "5", "/", "+"]
+    >>> evaluate_postfix(array)
+    6
+    >>> array = []
+    >>> evaluate_postfix(array)
+    0
+    """
+    operations = {'+', '-', '*', '/'}
+    stack = []
+
+    for token in postfix_notation:
+        if token in operations:
+            b, a = stack.pop(), stack.pop()
+            if token == "+":
+                stack.append(a + b)
+            elif token == "-":
+                stack.append(a - b)
+            elif token == "*":
+                stack.append(a * b)
+            else:
+                if a * b < 0 and a % b != 0:
+                    stack.append(a // b + 1)
+                else:
+                    stack.append(a // b)
+        else:
+            stack.append(int(token))
+
+    return stack.pop() if len(stack) != 0 else 0
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()