Strobogrammatic Number II -- LeetCode

A strobogrammatic number is a number that looks the same when rotated 180 degrees (looked at upside down).

Find all strobogrammatic numbers that are of length = n.

For example,
Given n = 2, return ["11","69","88","96"].

 class Solution {
 public:
     char num[] = {'', '', '', '', ''};
     char rotate(char num) {
         if (num == '' || num == '' || num == '')
             return num;
         else if (num == '') return '';
         else  return '';
     }
     void help(vector<string>& res, string& cand, int curInd) {
         if (cand.size() &  && curInd == (cand.size() - ) / ) {
             cand[curInd] = ''; res.push_back(cand);
             cand[curInd] = ''; res.push_back(cand);
             cand[curInd] = ''; res.push_back(cand);
         } else {
             int st = curInd ?  : ;
             for (; st < ; st++) {
                 cand[curInd] = num[st];
                 cand[cand.size() - curInd - ] = rotate(num[st]);
                 if (curInd == (cand.size() - ) / ) res.push_back(cand);
                 else help(res, cand, curInd + );
             }
         }
     }
     vector<string> findStrobogrammatic(int n) {
         vector<string> res;
         if (n < ) return res;
         string cand(n, '');
         help(res, cand, );
         return res;
     }
 };