Basic Calculator (with and without parentheses)

Requirements

Implement calculate(s: str) -> int over an expression string. Variants asked, easiest first:

1. No parentheses, +/-/*//: operator precedence matters but no nesting. Single pass with a stack of running terms.
2. With parentheses: same operators plus ( and ), including nested. Handle unary minus at the start of an expression or right after (.
3. 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.