Use a stack to keep track of the current result and the sign before a parenthesis. When encountering '(', push the current result and sign onto the stack, then reset them for the new sub-expression.

When encountering ')', pop the stack to retrieve the previous result and sign, then combine them with the current result (e.g., `result = prev_result + sign * current_result`).

Handle multi-digit numbers by accumulating digits until a non-digit character is encountered, then convert the accumulated string to an integer and apply the current sign.