224. Basic Calculator

class Solution {
    public int calculate(String s) {
        // init
        int sign = 1;
        int res = 0;
        
        LinkedList<Integer> stack = new LinkedList<>();
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            if (Character.isDigit(c)) {
                // the current digit
                int temp = c - '0';
                // if the following characters are digit, add them all
                while (i + 1 < s.length() && Character.isDigit(s.charAt(i + 1))) {
                    temp = temp * 10 + (s.charAt(i + 1) - '0');
                    i++;
                }
                res += sign * temp;
            } else if (c == '+') {
                sign = 1;
            } else if (c == '-') {
                sign = -1;
            } else if (c == '(') {
                // (BOTTOM) res ->         (TOP)
                // (BOTTOM) res -> sign -> (TOP)
                stack.addLast(res);
                stack.addLast(sign);
                // reset sign and result
                sign = 1;
                res = 0;
            } else if (c == ')') {
                // res = sign * current_res + previous_res
                res = stack.removeLast() * res + stack.removeLast();
            } else {
                // space: ignore it
            }
        }
        return res;
    }
}