Remove Invalid Parentheses

public class Solution {    int max = 0;    public List<String> removeInvalidParentheses(String s) {        if (s == null) {            throw new IllegalArgumentException("haha");        }        List<String> res = new LinkedList<>();        helper(res, s, "", 0, 0);        if (res.size() == 0) {            res.add("");        }        return res;    }        private void helper(List<String> res, String left, String right, int leftCount, int leftMax) {        if (left.length() == 0) {            if (leftCount == 0 && right.length() > 0) {                if (leftMax > max) {                    max = leftMax;                }                if (leftMax == max && !res.contains(right)) {                    res.add(right);                }            }            return;        }        if (left.charAt(0) == '(') {            helper(res, left.substring(1), right + '(', leftCount + 1, leftMax + 1);            helper(res, left.substring(1), right, leftCount, leftMax);        } else if (left.charAt(0) == ')') {            if (leftCount > 0) {                helper(res, left.substring(1), right + ')', leftCount - 1, leftMax);            }            helper(res, left.substring(1), right, leftCount, leftMax);        } else {            helper(res, left.substring(1), right + left.charAt(0), leftCount, leftMax);        }    }}