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

O(N) solution: use a hash table. H[i] = list of all integers in the three arrays that map to i.

For all H[i] > 1 check if three of its values are the same. If yes, you have your solution. You can do this check with the naive solution even, it should still be very fast, or you can sort those H[i] and then it becomes trivial.

If your numbers are relatively small, you can use H[i] = k if i appears k times in the three arrays, then the solution is the i for which H[i] = 3. If your numbers are huge, use a hash table though.

You can extend this to work even if you can have elements that can be common to only two arrays and also if you can have elements repeating elements in one of the arrays. It just becomes a bit more complicated, but you should be able to figure it out on your own.