如何创建字符串的所有元音组合并将每个组合添加到ArrayList

import java.util.ArrayList;
import java.util.List;

public class VowelCombinations {
    public static void main(String[] args) {
        String str = "aeiou";
        List<String> combinations = generateVowelCombinations(str);
        System.out.println(combinations);
    }

    public static List<String> generateVowelCombinations(String str) {
        List<String> combinations = new ArrayList<>();
        backtrack(combinations, str, new StringBuilder(), 0);
        return combinations;
    }

    private static void backtrack(List<String> combinations, String str, StringBuilder sb, int index) {
        if (index == str.length()) {
            combinations.add(sb.toString());
            return;
        }

        char c = str.charAt(index);
        if (isVowel(c)) {
            sb.append(c);
            backtrack(combinations, str, sb, index + 1);
            sb.deleteCharAt(sb.length() - 1);
        }
        backtrack(combinations, str, sb, index + 1);
    }

    private static boolean isVowel(char c) {
        return c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u';
    }
}