224. Basic Calculator

/**
TC: O(N)
SC: O(N)
*/
class Solution {
    public int calculate(String s) {
        if(s == null || s.length() == 0)
            return 0;

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