Eval RPN

Python data engineering interview problem. Difficulty: intermediate. Pattern: Stacks. About 14 minutes. Part of the Pro drill bank.

Evaluate a Reverse Polish Notation expression. Treat this as a production helper: match the contracted return shape, including empty and duplicate inputs.

Some systems store expressions in postfix form so an operator always follows the values it needs. A stack can evaluate this form from left to right. Evaluate tokens containing integers and +, -, *, /. When dividing, truncate toward zero, not toward negative infinity. Write eval_rpn(tokens: list) -> int. ['2', '1', '+', '3', '*'] first makes 3, then multiplies by 3, giving 9. The starter file already has the function name and harness print lines. Keep them and fill only the function body.

Requirements

• Push numeric tokens.
• For each operator, pop the right operand then the left operand.
• Return the final integer and truncate division toward zero.

Constraints

• The expression is valid and uses the four named operators.
• Tokens are strings and the return type is int.

Examples

Input: eval_rpn(['2', '1', '+', '3', '*']) Output: 9 (2+1)*3 = 9.

Topics: lakebench, python, rpn, eval.

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