Subsets problem on LeetCode

Subsets.java

import java.util.Arrays;
import java.util.ArrayList;

public class Subsets{
    public ArrayList<ArrayList<Integer>> subsets(int[] S) {
        // sort the array first
        Arrays.sort(S);
        ArrayList<ArrayList<Integer>> results = new ArrayList<ArrayList<Integer>>();
        ArrayList<Integer> emptySet = new ArrayList<Integer>();
        results.add(emptySet);
        // get the real subsets
        results = subsets(results, S, 0);
        return results;
    }
    
    private ArrayList<ArrayList<Integer>> subsets(ArrayList<ArrayList<Integer>> results, int[] S, int currentInd){
        if (S == null) return results;
        if (S.length == 1 || currentInd == S.length - 1){
            ArrayList<Integer> set = new ArrayList<Integer>();
            set.add(S[currentInd]);
            results.add(set);
            return results;
        }
        int temp = S[currentInd];
        results = subsets(results, S, currentInd + 1);
        // use this constructor instead. Better not to create an empty ArrayList and copy each element's clone to it.
        ArrayList<ArrayList<Integer>> newSets = new ArrayList<ArrayList<Integer>>(results);
        for(ArrayList<Integer> former: results){
            ArrayList<Integer> present = new ArrayList<Integer>(former);
            present.add(0, temp);
            newSets.add(present);
        }
        return newSets;
    }
}