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 * 105`
- `s` 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).
- There will be no two consecutive operators in the input.
- Every number and running calculation will fit in a signed 32-bit integer.