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:

• <code>1 <= s.length <= 3 * 10⁵</code>

• 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.