동작은 다 맞게 돌아가는데 arrayList에 값이 항상 이상하게 들어가네요.... 예제 입력시 출력은
INVALID HYPOTHESIS
INVALID HYPOTHESIS
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로 뜹니다 .. .ㅠㅠ 해결좀부탁드릴게요

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Scanner;

public class DICTIONARY {
static boolean[][] adj;
static boolean[] visited;
static List arrayList;

public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int testcase = sc.nextInt();
    for (int t = 0; t < testcase; t++) {
        int N = sc.nextInt();
        adj = new boolean[26][26];
        visited = new boolean[26];

        arrayList = new <Integer>ArrayList();
        String[] strArr = new String[N];
        for (int i = 0; i < N; i++) {
            strArr[i] = sc.next();
        }
        makeGraph(strArr);
        sort();
        if (!arrayList.isEmpty()) {
            for (int i = 0; i < arrayList.size(); i++)
                System.out.print((char) (arrayList.get(i) + 'a'));
        }
        else
            System.out.print("INVALID HYPOTHESIS");
        System.out.println();

    }
}

public static void makeGraph(String[] strArr) {
    int len = 0;
    for (int j = 1; j < strArr.length; j++) {
        int i = j - 1;
        len = min(strArr[i].length(), strArr[j].length());
        for (int k = 0; k < len; k++) {
            if (strArr[i].charAt(k) != strArr[j].charAt(k)) {
                int a = strArr[i].charAt(k) - 'a';
                int b = strArr[j].charAt(k) - 'a';
                adj[a][b] = true;
                break;
            }
        }
    }
}

public static void dfs(int here) {
    visited[here] = true;
    for (int i = 0; i < visited.length; i++) {
        if (adj[here][i] == true && visited[i] == false) {
            dfs(i);
        }
    }
    arrayList.add(here);

}

public static void sort() {
    for (int i = 0; i < visited.length; i++)
        if (visited[i] == false)
            dfs(i);
    Collections.reverse(arrayList);

    for (int i = 0; i < adj[0].length-1; i++)
        for (int j = i + 1; j < adj[0].length; j++)
            if (adj[j][i] == true)
                arrayList = new ArrayList();

}

public static int min(int a, int b) {
    return a > b ? b : a;
}

}