You want to find people who have as much exposure to different cultures as yourself.
uncommon_citieshelper that takes the cities you have visited (
my_cities) and the cities the other person has visited (
other_cities) and returns the number of cities that both sequences do NOT have in common.
So given [A B C] and [B C D] it should return 2 because only A and D are different.
You can loop through both sequences but maybe there is a more concise way to do it?