string frequencySort(string s) {

if(s.size() == 0) return "";

unordered_map<char,int> counter;

for(auto x : s){

counter[x]++;

}

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

[&](char a, char b){return counter[a] > counter[b] || (counter[a] == counter[b] && a < b);}

);

return s;

}