public:

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

vector<string> res;

unordered_map<string, int> map; //key is the string with sorted characters, value is the first index of such string

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

string s = strs[i];

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

if (map.find(s) == map.end()) {

// first occurance, put it in the map

map[s] = i;

} else {

// find such string more than once, put it the result

res.push_back(strs[i]);

int pos = map[s];

if (pos != -1) {

res.push_back(strs[pos]);

map[s] = -1; // mark the first string in map's pos "-1" to avoid it to be added to the result again

}

}

}

return res;

}