Sorting Elements of an Array by Frequency

#include<bits/stdc++.h>
using namespace std;
bool compare(pair<int,int> a,pair<int,int> b){
    if(a.first!=b.first) return a.first>b.first;
    else return a.second<b.second;
}
int main(){
    int t;cin>>t;
    while(t--){
        int n;cin>>n;
        unordered_map<int,int> mp;
        for(int i=0;i<n;i++){int a;cin>>a;mp[a]++;}
        vector<pair<int,int>> arr;
        for(auto itr:mp){
            arr.push_back(make_pair(itr.second,itr.first));
        }
        sort(arr.begin(),arr.end(),compare);
        for(auto itr:arr){
            for(int i=0;i<itr.first;i++) cout<<itr.second<<" ";
        }
        cout<<endl;
    }
    return 0;
}