sort_by_frequency.cpp

#include<bits/stdc++.h>
using namespace std;
#define tr(c,it)\
       for(typeof(c.begin()) = c.begin() ; it != c.end() ; it++)
#define all(c) (c).begin(),(c).end()
#define pb push_back
#define present(c,x) (c.find(x) != c.end())
bool mycomp(pair<int, int> p1, pair<int, int> p2){
    if(p1.first == p2.first)
        return(p1.second > p2.second);
    else return(p1.first<p2.first);
}
int main()
 {
	//code
	int t;
	cin>>t;
	while(t--)
	{
	    int n, i;
	    cin>>n;
	    vector<int> v1(n);
	    map<int, int> m1;
	    for(i=0 ; i<n ; i++)
	    {
	        cin>>v1[i];
	        if(present(m1, v1[i]))
	            m1[v1[i]] = m1[v1[i]] + 1;
	        else
	            m1[v1[i]] = 1;
	    }
	    map<int, int>::iterator it = m1.begin();
	    vector<pair<int, int> > v2;
	    for(; it != m1.end() ; it++)
	    {
	        v2.pb(make_pair(it->second, it->first));
	    }
	    
	    sort(v2.rbegin(), v2.rend(), mycomp);
	    int x = v2.size();
	    for(i=0 ; i<x ; i++)
	    {
	        int frequency = v2[i].first;
	        int value = v2[i].second;
	        for(int j=0 ; j<frequency ; j++)
	        {
	            cout<<value<<" ";
	        }
	    }
	    cout<<endl;
	    
	    
	}
	return 0;
}