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

static Map>> 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 src) {
    powerset(new LinkedList<>(), src);
}

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