Medium

Given a string, sort it in decreasing order based on the frequency of characters.

Example 1:

Input:"tree"Output:"eert"Explanation:'e' appears twice while 'r' and 't' both appear once.So 'e' must appear before both 'r' and 't'. Therefore "eetr" is also a valid answer.

 

Example 2:

Input:"cccaaa"Output:"cccaaa"Explanation:Both 'c' and 'a' appear three times, so "aaaccc" is also a valid answer.Note that "cacaca" is incorrect, as the same characters must be together.

 

Example 3:

Input:"Aabb"Output:"bbAa"Explanation:"bbaA" is also a valid answer, but "Aabb" is incorrect.Note that 'A' and 'a' are treated as two different characters.

class Solution {public:    string frequencySort(string s) {        int len = s.size();        // 建立一个hash表        unordered_map
       
         hash;        // 建立频率和字符的对应关系        vector
        
          vec(len+1, "");                for(auto ch: s) hash[ch]++;                for(auto val: hash) vec[val.second].append(val.second, val.first);                string ans;        for(int i = len; i > 0; i--)            ans += vec[i];        return ans;    }};