题：给定数组nums和整数k，求数组中 满足|nums[i]-nums[j]| = k（i<j）的数对数目。

解：1. 直接两重循环

class Solution:
    def countKDifference(self, nums: List[int], k: int) -> int:
        cnt = 0
        for i in range(len(nums)):
            for j in range(i+1,len(nums)):
                if abs(nums[j]-nums[i]) == k:
                    cnt += 1
        return cnt

2.哈希表

class Solution:
    def countKDifference(self, nums: List[int], k: int) -> int:
        cnt, ans = Counter(nums), 0
        for key in cnt:
            if (key + k) in cnt:
                ans += cnt[key] * cnt[key + k]                
        return ans