Number of Arithmetic Triplets

You're provided with a 0-indexed, strictly increasing integer array nums and a positive integer, diff. A triplet (i, j, k) is considered an arithmetic triplet if the following conditions hold:

1. i < j < k,
2. nums[j] - nums[i] == diff, and
3. nums[k] - nums[j] == diff.

Your task is to determine and print the number of unique arithmetic triplets in the given array.