Java DFS solution to Basic Calculator. Beats 96% of Java Submissions

Link to Question: https://leetcode.com/problems/basic-calculator/

Instructions: 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 * 10^5 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.