This is a little tricky as it requires a recursive function. To sniff out the pattern just write out a sample set of combinations. My biggest problem with it was that whether and how quickly I got the answer has little to no bearing on how I would handle the real day-to-day responsibilities of the job. It's ridiculous. Nobody codes low-level crap like that anymore anyway. Just go find it online, why re-invent the wheel. If he had asked me, I could have explained how I solved a real problem for a real client writing code to create baseball schedules that simultaneously meet nine conflicting and ranked criteria.