What intersection of arrays means?

@d40a The intersection of arrays is the array of values found in both arrays. In the problem 349 each value must be added once, i.e. the intersection of
[1,2,3,3,3]
and[3,3,3,3,3,4,5]
is[3]
. (Yes, the example in the text of the problem doesn't look selfexplanatory for me too, and the example in the problem 350 is even worse.)