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`.

Example 1:

    Input: nums = [3,1,4,1,5], k = 2
    Output: 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.

Example 2:

    Input: nums = [1,2,3,4,5], k = 1
    Output: 4
    Explanation: There are four 1-diff pairs in the array, (1, 2), (2, 3), (3, 4) and (4, 5).

Example 3:

    Input: nums = [1,3,1,5,4], k = 0
    Output: 1
    Explanation: There is one 0-diff pair in the array, (1, 1).

Constraints:

- `1 <= nums.length <= 104`
- `-107 <= nums[i] <= 107`
- `0 <= k <= 107`