Well either I suppose. For the subset problem as you say a filter critera can be applied while looping through the collection. And that should be it. Now if the question was multiple criteria I could see the solution diverging rapidly. If the filter can be expressed as a rank we could sort the collection first and then binary search through it to find the boundaries. I suppose I'm just trying to imagine some way an interviewee may turn that into two loops, or three loops... more?