I think that the array list version is easier to read and in general a collection is going to be easier to work with in Java.

The requirements don't mention that speed or memory are limited, so I didn't try to optimize my code for that. I think that in general best practices would say write the most readable and resuable code and if performance becomes an issue use a tool (profiler) to identify slow points (since the optimizer might actually correct something you assume is a problem).

You spend too much processing power creating an ArrayList to only convert it to an array. Why not declare the array?

I don't know... this would be a very, very minor speed improvement given that this algorithm is the worst in terms of speed.

You could compress lines 2 and 3 into one with one 'or' statement and improve speed.

Voted this one up for being more efficient than the indexOf solution.

I agree. All 'indexOf' type functions search the whole array for the supplied element and return the index it's currently in, like a for loop. Since you already go through numbers the size of the array (O(n)) and for each one you do the same with indexOf, you get O(n^2). The reduce solution, however, goes through the array only once, therefore being a O(n) solution.