Group Shifted Strings

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"]
]

Analyse: keep the diff between each character in the string. Put the strings with same diff in the same vector.

Runtime: 6ms

class Solution {
public:
    vector<vector<string>> groupStrings(vector<string>& strings) {
        unordered_map<string, vector<string> > groups;
        for (string s : strings) {
            if (s.size() == 1) {
                groups["-"].push_back(s);
            } else {
                string tempGap;
                for (int i = 1; i < s.size(); i++) {
                    if (s[i] > s[i - 1])
                        tempGap += to_string(s[i] - s[i - 1]);
                    else
                        tempGap += to_string(26 + (s[i] - s[i - 1]));
                }
                groups[tempGap].push_back(s);
            }
        }
        
        vector<vector<string> > result;
        for (auto ite = groups.begin(); ite != groups.end(); ite++) {
            vector<string> tempStrings = ite->second;
            result.push_back(vector<string>());
            for (auto s : tempStrings) {
                result.back().push_back(s);
            }
        }
        return result;
    }
};