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().
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 * 105s 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).