Sorting by character frequency

A daily topic ing, Today is a day medium topic 451. Sort Characters By Frequency

class Solution {
    
public:
	string frequencySort(string s) {
    
		unordered_map<char, int> umap;
		for (int i = 0; i < s.size(); i++) {
    
			umap[s[i]]++;
		}
		priority_queue<pair<int, char>> pq;
		for (auto p : umap) {
    
			pq.push(pair<int, char>(p.second, p.first));
		}
		string ret;
		int len = pq.size();
		while (len--) {
    
			auto p = pq.top();
			int cnt = p.first;
			char ch = p.second;
			while (cnt--) {
    
				ret.push_back(ch);
			}
			pq.pop();
		}
		return ret;
	}
};