[LeetCode] Remove Invalid Parentheses

 1 class Solution {
 2 public:
 3     vector<string> removeInvalidParentheses(string s) {
 4         vector<string> parens;
 5         queue<string> candidates;
 6         unordered_set<string> visited;
 7         candidates.push(s);
 8         visited.insert(s);
 9         bool found = false;
10         while (!candidates.empty()) {
11             string now = candidates.front();
12             candidates.pop();
13             if (isValid(now)) {
14                 parens.push_back(now);
15                 found = true;
16             }
17             if (!found) {
18                 int n = now.length();
19                 for (int i = 0; i < n; i++) {
20                     if (now[i] == '(' || now[i] == ')') {
21                         string next = now.substr(0, i) + now.substr(i + 1);
22                         if (visited.find(next) == visited.end()) {
23                             candidates.push(next);
24                             visited.insert(next);
25                         }
26                     }
27                 }
28             }
29         }
30         return parens;
31     }
32 private:
33     bool isValid(string& s) {
34         int c = 0, n = s.length();
35         for (int i = 0; i < n; i++) {
36             if (s[i] == '(') c++;
37             if (s[i] == ')' && !c--) return false;
38         }
39         return !c;
40     }
41 };