C++ :

#include <iostream>
#include <string>
#include <cstdio>
using namespace std; 
 
bool isSubStr(const std::string& str, int index1, int index2, int len)
{
    for (int i(0); i != len; ++i)
    {
        if (str[index1 + i] != str[index2 + i])
            return false;
    }
 
    return true;
}
 
// 在串中找以index开始，长度为len的串的数量
int repeat(const std::string& str, int index, int len)
{
    int rep(1);
 
    for (unsigned i(index + 1); i <= str.size() - len; ++i)
    {
        if (isSubStr(str, index, i, len))
            ++rep;
    }

    return rep;
}
 
int main()
{
    //freopen("1.txt","r",stdin);
	//freopen("2.txt","w",stdout); 
	
	int L;
    string S;
	
	int M;
 	cin >> M;
 	while(M--) {
 		cin >> L >> S;
 
    	int maxRep(0), maxIndex(0), maxLen(0);
 
    	// 子串长度
    	for (int len(S.size() - 1); len != L-1; --len)
    	{
        	for (unsigned i(0); i != S.size() - len; ++i)
        	{
          	  	// 从当前下标开始, len长度的子串
            	int rep = repeat(S, i, len);
 
            	if (maxRep < rep)
            	{
                	maxRep = rep;
                	maxIndex = i;
                	maxLen = len;
            	}
        	}
    	}
    	cout << S.substr(maxIndex, maxLen);
    	
    	cout << endl;
 	} 
    return 0;
}