Generating power set recursively without any loops

Question

How do you write a recursive method PowerSet(String input) that prints out all possible combinations of a string that is passed to it?

For example: PowerSet(\"abc\")

Answer 1

A recursive version of the generic solution proposed by João Silva :

public static  Set> powerSet2(Set originalSet) {
    Set> sets = new HashSet>();
    if (originalSet.isEmpty()) {
        sets.add(new HashSet());
        return sets;
    }
    List list = new ArrayList(originalSet);
    T head = list.get(0);
    Set rest = new HashSet(list.subList(1, list.size()));
    addSets(sets, powerSet(rest), head);
    return  sets;
}

private static  void addSets(Set> sets, Set> setsToAdd, T head) {
    Iterator> iterator = setsToAdd.iterator();
    if (iterator.hasNext()) {
        Set set = iterator.next();
        iterator.remove();
        Set newSet = new HashSet();
        newSet.add(head);
        newSet.addAll(set);
        sets.add(newSet);
        sets.add(set);
        addSets(sets, setsToAdd, head);
    }
}

I extract the recursive addSets method to transform the original for loop:

for (Set set : powerSet(rest)) {
    Set newSet = new HashSet();
    newSet.add(head);
    newSet.addAll(set);
    sets.add(newSet);
    sets.add(set);
}