public class Solution
extends Object
224 - Basic Calculator\.
Hard
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.