Heap-451. Sort Characters By Frequency

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=;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=;i<h.size();i++)
        {
            string t(h[i].second,h[i].first);
            ans+=t;
        }
        return ans;
    }
};