remove invalid parentheses

class Solution {
public:
    vector<string> removeInvalidParentheses(string s) {
        unordered_set<string> ret;
        int left = 0;
        int right = 0;
        for (char c : s) {
            if (c == '(') left++;
            if (c == ')') {
                if (left) left--;
                else right++;
            }
        }
        helper(s, 0, left, right, 0, "", ret);
        return vector<string>(ret.begin(), ret.end());
    }
    void (string s, int index, int left, int right, int pair, string path, unordered_set<string> &ret) {
        if (index == s.size()) {
            if (left == 0 && right == 0 && pair == 0) {
                ret.insert(path);
            }
            return;
        }
        if (s[index] != '(' && s[index] != ')') {
            helper(s, index+1, left, right, pair, path+s[index], ret);
        } else {
            if (s[index] == '(') {
                if (left) {
                    helper(s, index+1, left-1, right, pair, path, ret);
                }
                helper(s, index+1, left, right, pair+1, path+s[index], ret);
            }
            if (s[index] == ')') {
                if (right) {
                    helper(s, index+1, left, right-1, pair, path, ret);
                }
                if (pair) {
                    helper(s, index+1, left, right, pair-1, path+s[index], ret);
                }
            }
        }
    }
};