You should aim for a solution with O(n) time and O(n) space, where n is the size of the input array.

A brute force solution would be to check every element against every other element in the array. This would be an O(n^2) solution. Can you think of a better way?

Is there a way to check if an element is a duplicate without comparing it to every other element? Maybe there's a data structure that is useful here.

We can use a hash data structure like a hash set or hash map to store elements we've already seen. This will allow us to check if an element is a duplicate in constant time.