Find the inserted element in the list

Question

In a recent interview I was asked:

If you have 2 lists

listA listB

Each are the size 1000 and contain the same elements 1 - 1000. If an element, N is added to listB how can you determine the value of that element?

I responded correctly by saying to subtract listB from listA and the remainder would be the value.

Then he said what if we just have listB after N was added. How would you determine the value that was added?

I failed to answer this!!!! I should know it but I just cant think of it. The hint he gave was to do something similar as I did in the first problem.

Any suggestions?

Answer 1

You can exclusive XOR both lists and whatever is new (N) you will have it. This is the answer to the first question.