Given a string s representing a valid expression, implement a basic calculator to evaluate it, and return the result of the evaluation.
Note: You are not allowed to use any built-in function which evaluates strings as mathematical expressions, such as eval().
1 <= s.length <= 3 * 10^5s consists of digits, '+', '-', '(', ')', and ' 's represents a valid expression'+' is not used as a unary operation (i.e., "+1" and "+(2 + 3)" is invalid)'-' could be used as a unary operation (i.e., "-1" and "-(2 + 3)" is valid)s = "1 + 1"2s = " 2-1 + 2 "3s = "(1+(4+5+2)-3)+(6+8)"23