Given an array of integers nums and an integer k, return the number of unique k-diff pairs in the array.
A k-diff pair is an integer pair (nums[i], nums[j]), where the following are true:
0 <= i, j < nums.length
i != j
nums[i] - nums[j] == k
Notice that |val| denotes the absolute value of val.
Input: nums = [3,1,4,1,5], k = 2
Explanation: There are two 2-diff pairs in the array, (1, 3) and (3, 5).
Although we have two 1s in the input, we should only return the number of unique pairs.
Input: nums = [1,2,3,4,5], k = 1
Explanation: There are four 1-diff pairs in the array, (1, 2), (2, 3), (3, 4) and (4, 5).
Input: nums = [1,3,1,5,4], k = 0
Explanation: There is one 0-diff pair in the array, (1, 1).