Pairs That Differ by K

Write count_k_diff_pairs(nums, k) that counts the different pairs of values (a, b) in nums with b - a == k, where a and b come from two different positions in the list.

Pairs are counted by their values, not their positions: (1, 3) counts once, however many 1s and 3s there are. k is never negative, and when k is 0 a pair needs a value that appears at least twice.

Examples

count_k_diff_pairs([3, 1, 4, 1, 5], 2)  # returns 2
count_k_diff_pairs([1, 2, 3, 4, 5], 1)  # returns 4
count_k_diff_pairs([1, 3, 1, 5, 4], 0)  # returns 1

In the first example the pairs are (1, 3) and (3, 5); the second 1 does not make a new pair.

Constraints

• 0 <= len(nums) <= 100,000
• Values are between -10**7 and 10**7
• 0 <= k <= 10**7