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class Solution {
public:
    vector<vector<string>> groupStrings(vector<string>& strings) {
        unordered_map<string, vector<string> > mp;
        // Have strings pushed back to distance hash.
        for (string  s : strings)
            mp[shift(s)].push_back(s);

        // Just return the group of vector of strings.
        vector<vector<string> > groups;
        for(auto m : mp)
            groups.push_back(m.second);
        return groups;
    }
private:
    string shift(string& s) {
        // get the difference string.
        string t;
        int n = s.length();
        for(int i = 1; i < n; i++) {
            int diff = s[i] - s[i - 1];
            if(diff < 0) diff += 26;
            t += 'a' + diff;
        }
        return t;
    }
};