SAP XEP THEO CHU SO

#include <iostream>
#include <cmath>
#include <string>
#include <vector>
#include <algorithm>
using namespace std;
int SoSanh(string X, string Y){
    string XY = X.append(Y);
    string YX = Y.append(X);
    if(XY.compare(YX)>0){
        return 1;
    }
    return 0;
}
void SapXep(vector<string>arr){
    sort(arr.begin(), arr.end(), SoSanh);
    for(int i = 0; i<arr.size(); i++){
        cout << arr[i];
    }
}
int main(){
    int t;
    cin >> t;
    while(t--){
        int n;
        cin >> n;
        string str;
        vector<string> arr;
        for(int i = 0; i<n; i++){
            cin >> str;
            arr.push_back(str);
        }
        SapXep(arr);
        cout << endl;
    }
}