Given an int[] and a target value, return the number of all unique index pairs [i, j] where arr[i] + arr[j] == target and i < j. e.g. twoSumPairs(new int[]{2,7,4,1,3}, 5) has two valid pairs: index 0 and index 4, as well as index 2 and index 3. It will return 2. Use a single loop to achieve O(n) time.