subsequence of String

public class Solution{
    public static List<String> subsequenceOfString(String s) {
        if (s == null || s.length() == 0) {
            return new LinkedList<>();
        }

        Set<String> set = new HashSet<>();
        helper(s, set, new StringBuilder(), 0);
        List<String> res = new LinkedList<>(set);

        return res;
    }

    public static void helper(String s, Set<String> res, StringBuilder sb, int pos) {
        if (sb.length() != 0) {
            res.add(sb.toString());
        }

        for (int i = pos; i < s.length(); i++) {
            sb.append(s.charAt(i));
            helper(s, res, sb, i + 1);
            sb.deleteCharAt(sb.length() - 1);
        }
    }
    public static void main(String[] args){

        String s = "hello";
        Solution sol = new Solution();

        System.out.println(sol.subsequenceOfString(s));

    }
}