1679. Max Number of K-Sum Pairs

T: O(n)
S: O(n)

```java
class Solution {
    public int maxOperations(int[] nums, int k) {
        Map<Integer, Integer> map = new HashMap<>();
        int result = 0;
        for (int num : nums) {
            if (map.containsKey(k - num)) {
                result++;
                int count = map.get(k - num) - 1;
                if (count == 0) {
                    map.remove(k - num);
                } else {
                    map.put(k - num, count);
                }
            } else {
                map.put(num, map.getOrDefault(num, 0)+1);
            }
        }
        return result;
    }
}

/***
T: O(n)
S: O(n)
how many k sum pair in this array



use hashmap
4
  3
  3 -> 1
3
1


4 3 2 1

4
  3
  2 -> 1
1. -> 2

 */
```