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

Simple solution but with poor time complexity(2^n) is as following(just keep one thing in mind once we have to avoid(i.e. 0) and once we have to take it(i.e. 1):

public HashSet powerSet(int n) {
    return calcPowerSet(n-1, new HashSet(), new int[n]);
}

private HashSet calcPowerSet(int n, HashSet result, int []set) {
    if(n < 0) {
        result.add(set.clone());
        return null;
    }
    else {
        set[n] = 0;
        calcPowerSet(n-1, result, set);
        set[n] = 1;
        calcPowerSet(n-1, result, set);
        return result;
    }
}