Basic Calculator
Difficulty: Hard
Category: DSA
Topics: Math, String, Stack, Recursion
Asked at: Amazon, Roblox, Facebook, Indeed, Karat, Microsoft, Google, ByteDance, Uber, Wish
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.