Evaluate a string expression s containing non-negative integers and the operators +, -, *, / — no parentheses — where multiplication and division bind tighter than addition and subtraction, and operators of equal precedence apply left to right. The trick to handling precedence in one pass without ever building a parse tree: don't resolve an operator the moment it's seen — resolve the previous one, once its right-hand number is fully known. Track just one pending operator and the number being built. When a new operator (or the end of the string) arrives, apply the pending one: + and - simply push a (possibly negated) value; * and / instead pop whatever was pushed most recently and combine it immediately, so a tight-binding operation never lingers as two separate terms. By the end, every value left on the stack is already fully resolved, and the answer is just their sum.