Group Shifted Strings -- LeetCode

Given a string, we can "shift" each of its letter to its successive letter, for example: "abc" -> "bcd". We can keep "shifting" which forms the sequence:

"abc" -> "bcd" -> ... -> "xyz"

Given a list of strings which contains only lowercase alphabets, group all strings that belong to the same shifting sequence.

For example, given: ["abc", "bcd", "acef", "xyz", "az", "ba", "a", "z"], 
A solution is:

[
  ["abc","bcd","xyz"],
  ["az","ba"],
  ["acef"],
  ["a","z"]
]

 class Solution {
 public:
     string help(string word) {
         for (int i = , n = word.size(); i < n; i++) {
             word[i] = word[i] - word[] + 'a';
             while (word[i] < 'a') word[i] += ;
         }
         word[] = 'a';
         return word;
     }
     vector<vector<string>> groupStrings(vector<string>& strings) {
         unordered_map<string, int> dict;
         vector<vector<string> > res;
         for (int i = , n = strings.size(); i < n; i++) {
             string base = help(strings[i]);
             if (dict.count(base) == ) {
                 dict.insert(make_pair(base, res.size()));
                 vector<string> subGroup(, strings[i]);
                 res.push_back(subGroup);
             }
             else res[dict[base]].push_back(strings[i]);
         }
         return res;
     }
 };