Update evaluate_postfix_notations.py

data_structures/stacks/evaluate_postfix_notations.py

@@ -17,6 +17,10 @@ def evaluate_postfix(postfix_notation: list) -> int:
     9
     >>> evaluate_postfix(["4", "13", "5", "/", "+"])
     6
+    >>> evaluate_postfix(["2", "+"])
+    2
+    >>> evaluate_postfix(["5", "-"])
+    -5
     >>> evaluate_postfix([])
     0
     """
@@ -28,18 +32,28 @@ def evaluate_postfix(postfix_notation: list) -> int:
 
     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)
+            if len(stack) < 2:
+                operand = stack.pop()
+                if token == "-":
+                    operand = -operand
                 else:
+                    raise ValueError(f"Unrecognized {token = }")
+                stack.append(operand)
+            else:
+                b, a = stack.pop(), stack.pop()
+                if token == "+":
+                    stack.append(a + b)
+                elif token == "-":
+                    stack.append(a - b)
+                elif token == "*":
+                    stack.append(a * b)
+                elif token == "/":
+                    if b == 0:
+                        raise ValueError("Invalid expression: division by zero")
                     stack.append(a // b)
+                else:
+                    msg = f"Unrecognized {token = }"
+                    raise ValueError(msg)
         else:
             stack.append(int(token))