Generate a palindrome sequence of size N

GeneratePalindrome.java

import java.util.*;

public class GeneratePalindrome {
    public static List<String> palindromesOfLength(int N, Set<String> alphabet) {
        List<String> result = new ArrayList<String>();

        if(N == 1) {
            for(String letter : alphabet) {
                result.add(letter);
            }
        } else if(N > 1) {
            List<String> subsetPalindrome = palindromesOfLength(N - 2, alphabet);

            for(String s : alphabet) {
                if(! subsetPalindrome.isEmpty()) {
                    for(String palindrome : subsetPalindrome) {
                        result.add(s + palindrome + s);
                    }
                } else {
                    result.add(s + s);
                }
            }
        }
        return result;
    }

    public static void main(String[] args) {
        Set<String> alphabet = new TreeSet<String>();
        for(int i = 0; i < 10; i++) {
            alphabet.add(i + "");
        }

        List<String> result = GeneratePalindrome.palindromesOfLength(500, alphabet);
        for(String s : result) {
            System.out.println(s);
        }
    }

}