Given a string s representing an expression, implement a basic calculator to evaluate it. Example 1: Input: s = "1 + 1" Output: 2 Example 2: Input: s = " 2-1 + 2 " Output: 3 Example 3: Input: s = "(1+(4+5+2)-3)+(6+8)" Output: 23 Constraints: 1 <= s.length <= 3 * 105 s consists of digits, '+', '-', '(', ')', and ' '. s represents a valid expression.