I provided a couple of solutions using a simple Hash data structure and another solution using a Directed Acyclic Graph. I explained the pros and cons of each approach and the requirements that would need to be met in each solution. I had to terminate the interview early because the interviewer did not understand the nuance of my answers and became fixated on one aspect of one of the solutions I described. He refused to move on from the subject after I acknowledged his points, identified the misunderstanding and finally had to urgently ask that we move on.