that, given a non-empty zero-indexed array A of N integers, returns the first unique number in A. In other words, find the unique number with the lowest position in A. The function should return −1 if there are no unique numbers in A.

The most basic and straightforward approach: a double nested for loop, the external loop to scan each number and the internal loop to check if that number repeats (it's sufficient one another) in successive elements.