[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 = ; i < n; i++) {
                     if (now[i] == '(' || now[i] == ')') {
                         string next = now.substr(, i) + now.substr(i + );
                         if (visited.find(next) == visited.end()) {
                             candidates.push(next);
                             visited.insert(next);
                         }
                     }
                 }
             }
         }
         return parens;
     }
 private:
     bool isValid(string& s) {
         int c = , n = s.length();
         for (int i = ; i < n; i++) {
             if (s[i] == '(') c++;
             if (s[i] == ')' && !c--) return false;
         }
         return !c;
     }
 };