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

Description:

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

Example 1:

Input: [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:[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: [1, 3, 1, 5, 4], k = 0
Output: 1
Explanation: 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].

思路：题目意思是给出一组数组，要求查找绝对值差值为 k 的组合有多少个，可以使用 unordered_map 映射，直接查找某个值-k 是否存在，value 存储数组的值出现的次数，因为当 k=0 的时候，需要判断是否该值是否有不少于出现两次，除此之外我们就只用判断 k 值是否存在即可。特别注意 unordered_map 中 count 方法，只返回 1（存在）或 0（不存在）。

C++ 代码

class Solution {
public:
    int findPairs(vector<int>& nums, int k) {
      if (k < 0)
        return 0;
      unordered_map<int, int> umap;
      for (int num : nums)
        umap[num]++;
      
      int count = 0;
      for (auto u : umap){
        if (k == 0){
          if (u.second >=2 )
            count++;
        }
        else{
          count += umap.count(u.first - k);
        }
      }
      return count;
    }
};

运行时间：42ms
运行内存：12.5M