Arithmetic expression evaluation

Evaluate an arithmetic expression string containing non-negative integers, the binary operators + - * /, and parentheses. Standard precedence applies (* and / bind tighter than + and -); operators of equal precedence are left-associative. Division is integer division that truncates toward zero (like C). There may be spaces between tokens. The expression is always valid and contains no unary minus. Return the integer result. Assume no division by zero.