Implement a calculator that evaluates a string expression s containing non-negative integers, +, -, and matched parentheses — no multiplication, division, or spaces — respecting the fact that a sign in front of a parenthesized group applies to the whole group, not just its first term. Since there's no operator precedence to worry about here — only left-to-right evaluation and the way parentheses can flip a sign across an entire sub-expression — a single pass suffices, as long as entering and leaving a parenthesized group is handled correctly. Pushing the outer context (the result and sign pending before the group started) onto a stack the moment '(' is seen, and popping it back the moment ')' is seen, means never needing to know in advance where a group ends: the stack remembers exactly what to resume, exactly when it's needed.