[Codility] Algorithmic skills. FirstUnique

https://app.codility.com/programmers/trainings/4/first_unique/ A non-empty array A consisting of N integers is given. The unique number is the number that occurs exactly once in array A. For example, the following array A: A[0] = 4 A[1] = 10 A[2] = 5 A[3] = 4 A[4] = 2 A[5] = 10 contains two unique numbers (5 and 2). You should find the first unique number in A. In other words, find the unique nu..