#include <iostream>
#include <vector>
using namespace std;
int C;
int N;
int M;
char data[100 + 1][2 + 1];
char origin[26 + 1];
vector<char> result;
void Find(char &c)
{
    if (origin[c - 65] == '\0')
        return;
    bool isExist = false;
    char min = 99;
    for (int i = 0; i < M; i++)
    {
        if (data[i][1] != c)
        {

        }
        else
        {
            if (origin[data[i][0] - 65] != '\0')
            {
                if (min > data[i][0])
                {
                    min = data[i][0];
                }
                isExist = true;
            }
        }
    }
    if (isExist)
    {
        char p;
        for (int i = c + 1; i < min; i++)
        {
            p = i;
            Find(p);
        }
        result.push_back(min);
        origin[min - 65] = '\0';
        Find(c);
        c++;
    }
    else
    {
        result.push_back(c);
        origin[c - 65] = '\0';
    }

}
void Func()
{
    int originIndex = 0;
    char target = 65;

    int count = N;
    for (int i = 0; i < N; i++)
    {
        Find(target);
        target++;
    }
}

void SetUp()
{
    int alph = 65;
    for (int i = 0; i < N; i++)
    {
        origin[i] = alph++;
    }
}
int main()
{
    cin >> C;
    while (C--)
    {
        cin >> N;
        cin >> M;
        SetUp();
        for (int i = 0; i < M; i++)
        {
            cin >> data[i][0];
            cin >> data[i][1];
        }
        Func();
        for (int i = 0; i < N; i++)
        {
            cout << result.at(i);
        }
        cout << endl;
        result.clear();
    }
    return 0;
}