Find unique common element from 3 arrays

Question

Original Problem:
I have 3 boxes each containing 200 coins, given that there is only one person who has made calls from all of the three boxes and thus the

Answer 1

If you sort all the arrays first O(n log n) then it will be pretty easy to find the common element in less than O(n^3) time. You can for example use binary search after sorting them.