vector<vector<string>> groupAnagrams(vector<string>& strs) {

vector<vector<string>> res;

unordered_map<string, vector<string>> map;

if(strs.empty()) return res;

sort(strs.begin(), strs.end());

for(int i = 0; i < strs.size(); ++i){

string temp = strs[i];

if(!temp.empty())

sort(temp.begin(), temp.end());

map[temp].push_back(strs[i]);

}

for(unordered_map<string, vector<string>>::iterator it = map.begin(); it != map.end(); ++it)

res.push_back(it->second);

return res;

}