Basic Calculator I / II

LeetCode 224 / 227. Evaluate arithmetic expressions from a string. Stack-based evaluation with operator precedence; Meta-variant adds extra rules around eval order or unary operators. Requirements Parse and evaluate an arithmetic expression string with +, -, *, / and (LC 224) parentheses. A recent phone-screen variant explicitly removes parentheses and focuses on operator precedence plus invalid-expression handling, e.g. strings shaped like 10/2+1*3+5. Integer-only result; division truncates toward zero. Single-pass stack solution: push numbers, hold a sign / pending operator; multiply/divide eagerly to respect precedence. Examples "3+2*2" → 7. " 3/2 " → 1. "(1+(4+5+2)-3)+(6+8)" → 23. Notes Meta variants add extra rules: unary minus inside parens, operator-precedence variants, sometimes an extra ^ for exponent. Clarify the invalid-input contract before coding: return None, raise, or print an error. The phone-screen version leaves this open. Common bug: mishandling multi-digit numbers — accumulate the digit until the next non-digit char. A clean implementation uses one stack and a single sign carry, no recursion needed. Preparation Write LC 227 (no parens) from memory in <10 min. Layer parens on top (LC 224) using recursion or an inner stack. Practice the Meta-variant follow-ups: ^ precedence, -- unary, custom operator definitions.