排序之根据字符出现频率排序

每日一题ing，今天是个medium题451. Sort Characters By Frequency

class Solution {
public:
	string frequencySort(string s) {
		unordered_map umap;
		for (int i = 0; i < s.size(); i++) {
			umap[s[i]]++;
		}
		priority_queue> pq;
		for (auto p : umap) {
			pq.push(pair(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;
	}
};