Strobogrammatic Number II -- LeetCode

 1 class Solution {
 2 public:
 3     char num[5] = {‘0‘, ‘1‘, ‘6‘, ‘8‘, ‘9‘};
 4     char rotate(char num) {
 5         if (num == ‘0‘ || num == ‘1‘ || num == ‘8‘)
 6             return num;
 7         else if (num == ‘6‘) return ‘9‘;
 8         else  return ‘6‘;
 9     }
10     void help(vector<string>& res, string& cand, int curInd) {
11         if (cand.size() & 1 && curInd == (cand.size() - 1) / 2) {
12             cand[curInd] = ‘0‘; res.push_back(cand);
13             cand[curInd] = ‘1‘; res.push_back(cand);
14             cand[curInd] = ‘8‘; res.push_back(cand);
15         } else {
16             int st = curInd ? 0 : 1;
17             for (; st < 5; st++) {
18                 cand[curInd] = num[st];
19                 cand[cand.size() - curInd - 1] = rotate(num[st]);
20                 if (curInd == (cand.size() - 1) / 2) res.push_back(cand);
21                 else help(res, cand, curInd + 1);
22             }
23         }
24     }
25     vector<string> findStrobogrammatic(int n) {
26         vector<string> res;
27         if (n < 1) return res;
28         string cand(n, ‘0‘);
29         help(res, cand, 0);
30         return res;
31     }
32 };