"""
    We have a list of integers, where:

    The integers are in the range 1...n
    The list has a length of n...n+1

It follows that our list has at least one integer which appears at least twice.
 But it may have several duplicates, and each duplicate may appear more than twice.

Write a function which finds any integer that appears more than once in our list.

We're going to run this function on our new, super-hip Macbook Pro With Retina Display™.
 Thing is, the damn thing came with the RAM soldered right to the motherboard,
 so we can't upgrade our RAM. So we need to optimize for space!
"""