How to generate the power-set of a given List?

Question

I\'m trying to generate a collection of all 2^N - 1 possible combinations of a given List of length N. The collection will map the number of elements in a combination to an

Answer 1

public static List<String> getCombinationsLists(List<String> elements)
{
    
    //return list with empty String
    if(elements.size() == 0){
        List<String> allLists = new ArrayList<String>();
        allLists.add("");
        return allLists ;
    }

    String first_ele = elements.remove(0);
    List<String> rest = getCombinationsLists(elements);
    int restsize = rest.size();
    //Mapping the first_ele with each of the rest of the elements.
    for (int i = 0; i < restsize; i++) {
        String ele = first_ele + rest.get(i);
        rest.add(ele);
    }
   
    return   rest;
}

This Power set is one of the exercise in the book SICP "Structure and Interpretation of Computer Programming".Every Programmer should read it.

Answer 2

static Map<Integer, List<LinkedList<Integer>>> powerset = new HashMap<>();

public static void main(String[] args) throws IOException {
    powerset(Arrays.asList(1, 2, 3));
    for (Integer key : powerset.keySet()) {
        System.out.print(key + " -> ");
        System.out.println(Arrays.toString(powerset.get(key).toArray()));
    }
}

static void powerset(List<Integer> src) {
    powerset(new LinkedList<>(), src);
}

private static void powerset(LinkedList<Integer> prefix, List<Integer> src) {
    if (src.size() > 0) {
        prefix = new LinkedList<>(prefix); //create a copy to not modify the orig
        src = new LinkedList<>(src); //copy
        Integer curr = src.remove(0);
        collectResult(prefix, curr);
        powerset(prefix, src);
        prefix.add(curr);
        powerset(prefix, src);
    }
}

private static void collectResult(LinkedList<Integer> prefix, Integer curr) {
    prefix = new LinkedList<>(prefix); //copy
    prefix.add(curr);
    List<LinkedList<Integer>> addTo;
    if (powerset.get(prefix.size()) == null) {
        List<LinkedList<Integer>> newList = new LinkedList<>();
        addTo = newList;
    } else {
        addTo = powerset.get(prefix.size());
    }
    addTo.add(prefix);
    powerset.put(prefix.size(), addTo);
}

OUTPUT

1 -> [[1], [2], [3]]
2 -> [[2, 3], [1, 2], [1, 3]]
3 -> [[1, 2, 3]]