Add sample RPN solution

shared/notes-and-examples/2026.04.08/homework.py

@@ -13,9 +13,43 @@
 #
 # For help, email me@bchen.dev
 
-# Use this function definition:
+# Sample solution below
+
+# Facts:
+# The last element of the list must always be an operator; otherwise there is only one element
+
 def rpn(expression: list[str]) -> int:
-    return 0
+    operators = "+-*/"
+
+    def eval(expression: list[str]) -> tuple[int, int]:
+        """
+        Evaluate RPN and return an index value pointing to the first token in the expression.
+        """
+        for i in range(len(expression) - 1, -1, -1):
+            token = expression[i]
+
+            if token in operators:
+                right, right_token_index = eval(expression[:i])
+
+                # for left, we need the index of the token that comes after right
+                left, left_token_index = eval(expression[:right_token_index])
+
+                if token == "+":
+                    return left + right, left_token_index
+                elif token == "-":
+                    return left - right, left_token_index
+                elif token == "*":
+                    return left * right, left_token_index
+                else:
+                    return int(left / right), left_token_index
+            else:
+                # evaluate as a literal
+                literal = int(token)
+                return literal, i
+
+        return (0, 0)
+    
+    return eval(expression)[0]
 
 
 # The following test cases should pass (please copy this into your code):
@@ -27,6 +61,8 @@ tests = [
     (["50", "2", "/"], 25),
     (["5", "2", "/"], 2),  # for division, cast to int using int(num)
     (["5", "4", "3", "-", "*"], 5),
+    (["5", "4", "6", "2", "10", "+", "*", "-", "-"], 73),
+    (["3", "2", "+", "5", "*"], 25)
 ]
 for case in tests:
     result = rpn(case[0])