856. Score of Parentheses!

class Solution {
    public int scoreOfParentheses(String s) {
        return helper(s, 0, s.length()-1);
    }
    
    // from l to r (included), the substring is balanced
    private int helper(String s, int l, int r){
        // when r-l==1, "()"
        if(r-l==1) return 1;
        int balance = 0;
        for(int i = l; i<r; i++){
            if(s.charAt(i)=='(') balance++;
            if(s.charAt(i)==')') balance--;
            if(balance == 0){
                return helper(s, l,i) +helper(s, i+1,r);
            }
        }
        // full balanced
        return 2*helper(s, l+1, r-1);
    }
}