Basic Calculator with Postfix Notation

Implement a basic calculator for postfix notation. Details on operators, integer division, unary negatives, and invalid-token handling are thin, so clarify the grammar before coding. Requirements Input: an arithmetic expression in postfix / reverse Polish notation. Use a stack: push operands; when an operator arrives, pop the required operands, compute, and push the result. Clarify the operator set (+, -, *, /), integer vs floating division, negative numbers, whitespace / tokenization, and invalid expression handling. Notes This is the classic stack-evaluator shape, but the available prompt detail does not include a full grammar or sample expression. Do not assume LeetCode-style truncating division unless the interviewer confirms it. For binary operators, operand order matters: if the stack pops right then left, compute left op right. Preparation Write a reverse-Polish evaluator from scratch, including explicit invalid-token and too-few-operands checks. Practice explaining the tokenization contract before implementation: space-delimited tokens, signed integers, and division semantics.