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) {  
        string ans;  
        vector<pair<char,int
 
>> h;  
        map<char,int> f;  
        for(int i=0;i<s.size();i++)  
        {  
            f[s[i]]++;  
        }  
        for(auto it:f)  
        {  
                pair<char,int> p(it.first,it.second);  
                h.push_back(p);  
        }  
        sort(h.begin(),h.end(),[](pair
 
<char,int> a,pair<char,int> b){  
            return a.second>b.second;  
        });  
        for(int i=0;i<h.size();i++)  
        {  
            string t(h[i].second,h[i].first);  
            ans+=t;  
        }  
        return ans;  
    }  
};  

Heap-451. Sort Characters By Frequency