Obtaining a powerset of a set in Java

Question

The powerset of {1, 2, 3} is:

{{}, {2}, {3}, {2, 3}, {1, 2}, {1, 3}, {1, 2, 3}, {1}}

Let\'s say I have a Set in Java:<

Answer 1

Yes, it is O(2^n) indeed, since you need to generate, well, 2^n possible combinations. Here's a working implementation, using generics and sets:

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

And a test, given your example input:

 Set mySet = new HashSet();
 mySet.add(1);
 mySet.add(2);
 mySet.add(3);
 for (Set s : SetUtils.powerSet(mySet)) {
     System.out.println(s);
 }