249. Group Shifted Strings

class Solution {
public:
    vector<vector<string>> groupStrings(vector<string>& strings) {
        unordered_map<string, int> dict;
        vector<vector<string>> res;
        for (string &s : strings) {
            string tmp;
            for (int ch:s) tmp += ch - s[0] >= 0 ? (ch - s[0]) % 26 + 'a' : ch - s[0] + 26 + 'a' ;
            if (!dict.count(tmp)) {
                dict[tmp] = res.size();
                res.push_back({});
            }
            res[dict[tmp]].push_back(s);
        }
        return res;
    }
};