Given an unsorted array of integers, find the first pair with given sum in it. Return the index for these integers.
When the solution is called with the following input arr = [8, 7, 2, 5, 3, 1], sum = 10, it is expected to return [0, 2]. arr[0] holds the value 8 and arr[2] is 2. 8 + 2 = 10 which is the desired sum value.