Basic Calculator III

You are given a string `s` representing a valid mathematical expression. The expression may contain non-negative integers, the operators `'+'`, `'-'`, `'*'`, `'/'`, parentheses `'('`, `')'`, and spaces. Evaluate the expression and return its result as an integer. Note that integer division should truncate toward zero. You are **not allowed** to use any built-in function that evaluates strings as mathematical expressions (such as `eval()`). **Constraints:** - `1 <= s.length <= 10^4` - `s` consists of digits, operators `'+'`, `'-'`, `'*'`, `'/'`, parentheses `'('`, `')'`, and spaces. - The given expression is always valid. Return the integer result of evaluating the expression.