BasicCalculatorClean.java

 // The main idea of this is the left bracket might change the sign of a number, however, this does not seem to be very generalized
    // https://leetcode.com/problems/basic-calculator/discuss/62362/JAVA-Easy-Version-To-Understand!!!!!
    public class Solution {
        // 1-2-(4+5+2)-3
        public int calculate(String s) {
            int len = s.length(), sign = 1, result = 0;
            // Use one stack for numbers, for operators used the sign, + -> 1, - -> -1
            Stack<Integer> stack = new Stack<>();

            for (int i = 0; i < len; i++) {
                if (Character.isDigit(s.charAt(i))) {
                    int sum = s.charAt(i) - '0';
                    while (i + 1 < len && Character.isDigit(s.charAt(i + 1))) {
                        sum = sum * 10 + s.charAt(i + 1) - '0';
                        i++;
                    }
                    result += sum * sign;
                } else if (s.charAt(i) == '+')
                    sign = 1;
                else if (s.charAt(i) == '-')
                    sign = -1;
                else if (s.charAt(i) == '(') {
                    stack.push(result);
                    stack.push(sign);
                    result = 0;
                    sign = 1;
                } else if (s.charAt(i) == ')') {
                    // first pop is the sign, +1 or -1, second is the value
                    result = result * stack.pop() + stack.pop();
                }

            }
            return result;
        }
    }