#include<iostream>
#include<vector>
#include<string>
#include<algorithm>

using namespace std;

vector<vector<int>> adj; //인접행렬
vector<string> s;//string을 넣는 vector
vector<int> seen;//방문 여부
vector<int> list;

void dfs(int here) {//dfs탐색
    seen[here] = 1;
    for (int i = 0; i < adj.size(); i++) {
        if (adj[here][i] && !seen[i])
            dfs(i);
    }
    list.push_back(here);//방문한것을 list에 넣는다.
}

vector<int> dfsAll(void) {//컴포넌트를 모두 순회하기 위해서.
    int n = adj.size();
    seen = vector<int>(n, 0);//초기화를 진행한다(오버라이딩)
    list.clear();
    for (int i = 0; i < n; i++)
        if (!seen[i])
            dfs(i);
    reverse(list.begin(), list.end());

    for (int i = 0; i < n; i++) {
        for (int j = i + 1; j < n; j++) {
            if (adj[list[j]][list[i]]) return vector<int>();
        }
    }
    return list;
}

/*인접 행렬을 만드는 부분*/
void makegragh(const vector<string>& words) {
    adj = vector<vector<int>>(26, vector<int>(26, 0));//그래프 초기화를 실행

    for (int i = 1; i < words.size(); i++) {//j,i
        int j = i - 1;
        int len = min(words[i].size(), words[i].size());
        for (int k = 0; k < len; k++) {
            if (words[i][k] != words[j][k]) {
                int a = words[j][k] - 'a';
                int b = words[i][k] - 'a';
                adj[a][b] = 1;
                break;
            }
        }
    }
}



int main(void) {
    int T;
    int n;
    vector<int> l;
    cin >> T;
    for (int i = 0; i < T; i++) {
        //초기화 공정
        adj.clear();
        s.clear();
        l.clear();
        cin >> n;
        s = vector<string>(n);
        for (int j = 0; j < n; j++) {
            cin >> s[j];
        }
        makegragh(s);
        l = dfsAll();
        if (l == vector<int>())
            cout << "INVALID HYPOTHESIS" << endl;
        else {
            for (int k = 0; k < 26; k++)
                cout << (char)(l[k]+'a');
            cout << endl;
        }
    }
    return 0;
}

(C++ 소스 코드)