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

Example 3:

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

Note:

1. The pairs (i, j) and (j, i) count as the same pair.
2. The length of the array won't exceed 10,000.
3. All the integers in the given input belong to the range: [-1e7, 1e7].

这道题是让找数值对，使数值对中的数字差的绝对值为给定的k，但是数值对不能重复。这时首先想到遍历每个数字，看是否存在与该数字的差为k的数字，但是这样很容易超时，并且不好判断重复的数值对，所以应该想到用map，首先新建一个map去保存每个key出现的次数，k=0，则只需要统计出现次数大于等于2的，即map中的value大于等于2，否则的话，判断map中是否存在key+k的key1，用map.containsKey判断，代码如下：

       if (nums == null || nums.length == 0 || k < 0)   return 0;                    Map<Integer, Integer> map = new HashMap<>();          int count = 0;          for (int i : nums) {              map.put(i, map.getOrDefault(i, 0) + 1);          }                    for (Map.Entry<Integer, Integer> entry : map.entrySet()) {              if (k == 0) {                  //count how many elements in the array that appear more than twice.                  if (entry.getValue() >= 2) {                      count++;                  }               } else {                  if (map.containsKey(entry.getKey() + k)) {                      count++;                  }              }          }                    return count;  

map.getOrDefault(i,0)表示：如果map中有key为i则返回i对应的value，否则用默认值0；Map.Entry用于对map的迭代，Map是java中的接口，Map.Entry是Map的一个内部接口。Map提供了一些常用方法，如keySet(),entrySet()等，keySet()的返回值是Map中key的集合，entrySet()是返回一个Set集合，集合中是的map的键值对。