Evaluate a Four-Operator Expression

Implement a function that evaluates an arithmetic expression string containing non-negative integers, optional spaces, and the operators `+`, `-`, `*`, and `/`. There are no parentheses. Multiplication and division must be evaluated before addition and subtraction. Integer division should truncate toward zero. Function signature: `int evaluate(string s)` Examples: - `"3+2*2"` -> `7` - `" 14 - 3 / 2 "` -> `13` - `"18/3*2+5"` -> `17` Assume the input expression is always valid.