451. Sort Characters By Frequency

class Solution {
public:
    string frequencySort(string s) {
        unordered_map<char, int> freq;
        vector<string> bucket(s.size() + 1);

        for (char c : s) freq[c]++;

        for (auto kv : freq) 
            for (int i = 0; i < kv.second; i++) //
                bucket[kv.second] += kv.first;  //bucket[kv.second].append(kv.second, kv.first);

        string res;

        for (int i = s.size(); i > 0; i--)
            res += bucket[i];

        return res;
    }
};