Remove Invalid Parentheses

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public class Solution {    int max = 0;    public List<String> removeInvalidParentheses(String s) {        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 maxCount) {        if (left.length() == 0) {            if (leftCount == 0 && right.length() != 0) {                if (maxCount > max) {                    max = maxCount;                }                if (maxCount == max && !res.contains(right)) {                    res.add(right);                }            }            return;        }        if (left.charAt(0) == '(') {            helper(res, left.substring(1), right + '(', leftCount + 1, maxCount + 1);            helper(res, left.substring(1), right, leftCount, maxCount);        } else if (left.charAt(0) == ')') {            if (leftCount > 0) {                helper(res, left.substring(1), right + ')', leftCount - 1, maxCount);            }            helper(res, left.substring(1), right, leftCount, maxCount);        } else {            helper(res, left.substring(1), right + left.charAt(0), leftCount, maxCount);        }    }}

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