题目：

题解：

class Solution {
public:string frequencySort(string s) {unordered_map<char, int> mp;int maxFreq = 0;int length = s.size();for (auto &ch : s) {maxFreq = max(maxFreq, ++mp[ch]);}vector<string> buckets(maxFreq + 1);for (auto &[ch, num] : mp) {buckets[num].push_back(ch);}string ret;for (int i = maxFreq; i > 0; i--) {string &bucket = buckets[i];for (auto &ch : bucket) {for (int k = 0; k < i; k++) {ret.push_back(ch);}}}return ret;}
};