Code Craft: Basic Calculator (No Parentheses)

Phone-screen coding round. Implement a basic calculator that handles `+`, `−`, `×`, `÷` over integer operands. No parentheses. Standard operator-precedence evaluation. Requirements Input: a string expression containing non-negative integers, the four operators + − × ÷, and possibly whitespace. No parentheses. Output: the evaluated integer (truncate toward zero for division per LC convention; clarify rounding rule with interviewer). Operator precedence: × ÷ before + −. Left-to-right associativity within a precedence class. Notes Standard approach: single-pass tokenize + stack. Walk characters, build the current number. On hitting an operator (or end of string), apply the previous operator to the current number based on whether it was +, −, ×, or ÷: +: push num onto the stack. −: push −num. ×: pop top, push top × num. ÷: pop top, push top ÷ num (with the agreed rounding). At the end, sum the stack. Equivalent to LC 227 "Basic Calculator II" — drill that until automatic. Edge cases: leading whitespace, multi-digit numbers, division by zero (clarify expected behavior), negative results, very large numbers (use 64-bit if the interviewer pushes scale). Follow-up sometimes asked: extend to support parentheses (LC 224 "Basic Calculator"), or add unary minus. Preparation Drill LC 227 and LC 224 back-to-back; the stack pattern is the same skeleton with one extra recursive case for (. Pre-write the tokenizer + stack template in under 10 minutes. Have the Python integer-division convention story ready (// rounds toward negative infinity; LC expects truncation toward zero — use int(a / b) instead).