Basic Calculator (with and without parentheses)

Implement a string calculator supporting +, -, *, / on integers. Variants range from no-parentheses (operator precedence only) to full infix with parentheses and unary minus. Requirements Implement calculate(s: str) -> int over an expression string. Variants asked, easiest first: No parentheses, +/-/*//: operator precedence matters but no nesting. Single pass with a stack of running terms. With parentheses: same operators plus ( and ), including nested. Handle unary minus at the start of an expression or right after (. Full infix expression (reported as the toughest variant): includes spaces, multi-digit numbers, possible nested parentheses, and the interviewer expects you to handle every edge case before writing code. Common signatures: def calculate(s: str) -> int: ... Notes The canonical pattern is a one-pass scan with a stack of partially-evaluated terms and a prev_op register. On seeing +/-, push the current number with its sign; on *//, pop the top, combine, push back. For nested parentheses, push (result_so_far, sign_before_paren) onto a stack when you see ( and unwind on ). Integer division semantics: most variants want truncation toward zero, not floor — clarify with the interviewer because Python's // is floor division. A common follow-up: add support for unary -, then for ^ (right-associative exponent). The shunting-yard algorithm generalizes cleanly if asked to go further. Reported pitfall: candidates who jump straight to shunting-yard / two-stack approaches sometimes get stuck on edge cases like "3+ -2" or empty parens. The single-stack template is more forgiving. Preparation Write the no-parentheses version from scratch until you can do it in 10 minutes with full test cases. Then extend to support parentheses by adding the (value, sign) stack frame trick. Practice walking through "(2+3)*(4-1)" and "3+5/2" on the whiteboard — these are the standard sanity checks interviewers reach for. Have a clean test harness ready: at least one test per operator, one nested-parens test, one division-rounding test.