Basic Calculator

https://leetcode.com/problems/basic-calculator/description/

public int calculate(String s) {
    if(s == null) return 0;

    int result = 0;
    int sign = 1;
    int num = 0;

    Stack<Integer> stack = new Stack<Integer>();
    stack.push(sign);

    for(int i = 0; i < s.length(); i++) {
        char c = s.charAt(i);

        if(c >= '0' && c <= '9') {
            num = num * 10 + (c - '0');

        } else if(c == '+' || c == '-') {
            result += sign * num;
            sign = stack.peek() * (c == '+' ? 1: -1); 
            num = 0;

        } else if(c == '(') {
            stack.push(sign);

        } else if(c == ')') {
            stack.pop();
        }
    }

    result += sign * num;
    return result;
}