题目
Given an array of integers and an integer k, you need to find the number of unique k-diff pairs in the array. Here a k-diff pair is defined as an integer pair (i, j), where i and j are both numbers in the array and their absolute difference is k.

答案

class Solution {
    public int findPairs(int[] nums, int k) {
        if(k < 0) return 0;
        Map<Integer, Integer> map = new HashMap<Integer, Integer>();
        for(int num : nums) 
            map.put(num, (map.get(num) == null)? 1:map.get(num) + 1);
        int ans = 0;
        for(int num : map.keySet()) {
            Integer n = map.get(num);
            if(k == 0) {
                if(n > 1) ans++;
            }
            else {
                if(map.containsKey(num - k)) {
                    ans++;
                }
                /*
                You may search for num - k or num + k, but not both, otherwise you get duplicated pairs 
                if(map.containsKey(num + k)) {
                    ans++;
                }
                */
            }

        }
        return ans;
    }
}