[LeetCode] 532. K-diff Pairs in an Array

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.

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).

Example 4:

Input: nums = [1,2,4,4,3,3,0,9,2,3], k = 3
Output: 2

Example 5:

Input: nums = [-1,-2,-3], k = 1
Output: 2

Constraints:

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

数组中的K-diff数对。题意是给一个整数数组和一个整数K，请你求出数组中有多少对数字满足两个数字之间的差值是K。

思路是用hashmap记录每个数字和他们出现的次数。再次遍历数组，也要分两种情况讨论

如果K = 0，那么只要找到任何一个出现超过两次的数字，就res++
如果K > 0，那么当遍历到num[i]的时候，就去看hashmap中是否有nums[i] + k，有则res++

对于第二种情况，我第一次做的时候有试图去找nums[i] - k，后来发觉没必要。因为hashmap会遍历所有的key，所以结果不会丢失

时间O(n)

空间O(n)

Java实现

 1 class Solution {
 2     public int findPairs(int[] nums, int k) {
 3         // corner case
 4         if (nums == null || nums.length == 0) {
 5             return 0;
 6         }
 7 
 8         // normal case
 9         HashMap<Integer, Integer> map = new HashMap<>();
10         int res = 0;
11         for (int num : nums) {
12             map.put(num, map.getOrDefault(num, 0) + 1);
13         }
14 
15         for (Map.Entry<Integer, Integer> entry : map.entrySet()) {
16             if (k == 0) {
17                 if (entry.getValue() >= 2) {
18                     res++;
19                 }
20             } else {
21                 if (map.containsKey(entry.getKey() + k)) {
22                     res++;
23                 }
24             }
25         }
26         return res;
27     }
28 }

LeetCode 题目总结