"n" slots - continued..

A different java implementation, closer to being a good functional implementation, but not quite there:

public List<String> fillNSlots(int n){
        return fillNSlotsWithPrefix(0, n, "");
        
    }
    
    private List<String> fillNSlotsWithPrefix(int counter, int  n, String prefix){
        if (counter==n-1){
            return new ArrayList<String>(Arrays.asList(new String[]{prefix + "0", prefix + "1"})); 
        }else{
            List<String> result1 = fillNSlotsWithPrefix(counter+1, n , prefix + "0");
            result1.addAll(fillNSlotsWithPrefix(counter+1, n , prefix + "1"));
            return result1;
        }
    }

It is not quite there, as there is still a need to hold an intermediate state with a temporary variable, result1 above, which could have been avoided had List supported an API which adds an element and returns itself or returns a new list.

The following is an equivalent implementation in scala:

def fillNSlots(n:Int):List[String] = {
        fillNSlotsWithPrefix(0,n,"")
    }

    def fillNSlotsWithPrefix(counter:Int, n: Int, prefix:String): List[String] = {
        if (counter==(n-1))
            (prefix+"0")::(prefix+"1")::List()
        else
            fillNSlotsWithPrefix(counter+1, n, prefix+"0"):::fillNSlotsWithPrefix(counter+1, n, prefix+"1")
    }