301. Remove Invalid Parentheses

class Solution {
    public List<String> removeInvalidParentheses(String s) {
        List<String> res = new ArrayList<>();
        helper(s, 0, 0, res, new char[]{'(', ')'});
        return res;
    }
    
    private void helper(String s, int left, int right, List<String> res, char[] pars) {
        int stack = 0;
        int n = s.length();
        for (; right < n; right++) {
            char c = s.charAt(right);
            if (c == pars[0]) {
                stack++;
            } else if (c == pars[1]) {
                stack--;
            }
            
            if (stack < 0) break;
        }
        
        if (stack < 0) {
            for (; left <= right; left++) {
                char c = s.charAt(left);
                if (c != pars[1]) continue;
                if (left > 1 && s.charAt(left) == s.charAt(left - 1)) continue;
                helper(s.substring(0, left) + s.substring(left + 1), left, right, res, pars);
            }
        } else if (stack > 0) {
            helper(new StringBuilder(s).reverse().toString(), 0, 0, res, new char[]{')', '('});
        } else {
            res.add(pars[0] == '(' ? s : new StringBuilder(s).reverse().toString());
        }
    }
}