DAY 17

//SORT CHAR BY FREQUENCY
class Solution {
public:
    string frequencySort(string s) {
        auto cmp = [](const pair<char, int>& a, const pair<char, int>& b) {
            return a.second < b.second;
        };
        
        priority_queue<pair<char, int>, vector<pair<char, int>>, decltype(cmp)> pq(cmp);
        
        unordered_map<char, int> hm;
        
        for (char c : s) {
            hm[c]++;
        }
        
        for (const auto& entry : hm) {
            pq.push(make_pair(entry.first, entry.second));
        }
        
        string result = "";
        while (!pq.empty()) {
            pair<char, int> p = pq.top();
            pq.pop();
            result.append(p.second, p.first);
        }
        
        return result;
    }
};