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

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.