[LeetCode] Remove Invalid Parentheses

This problem can be solved very elegantly using BFS, as in this post.

The code is rewritten below in C++.

class Solution {
public:
    vector<string> removeInvalidParentheses(string s) {
        vector<string> parens;
        queue<string> candidates;
        unordered_set<string> visited;
        candidates.push(s);
        visited.insert(s);
        bool found = false;
        while (!candidates.empty()) {
            string now = candidates.front();
            candidates.pop();
            if (isValid(now)) {
                parens.push_back(now);
                found = true;
            }
            if (!found) {
                int n = now.length();
                for (int i = 0; i < n; i++) {
                    if (now[i] == '(' || now[i] == ')') {
                        string next = now.substr(0, i) + now.substr(i + 1);
                        if (visited.find(next) == visited.end()) {
                            candidates.push(next);
                            visited.insert(next);
                        }
                    }
                }
            }
        }
        return parens;
    }
private:
    bool isValid(string& s) {
        int c = 0, n = s.length();
        for (int i = 0; i < n; i++) {
            if (s[i] == '(') c++;
            if (s[i] == ')' && !c--) return false;
        }
        return !c;
    }
};