1679. Max Number of K-Sum Pairs

You are given an integer array `nums` and an integer `k`.

In one operation, you can pick two numbers from the array whose sum equals `k` and remove them from the array.

Return the maximum number of operations you can perform on the array.

Example 1:

```Input: nums = [1,2,3,4], k = 5
Output: 2
Explanation: Starting with nums = [1,2,3,4]:
- Remove numbers 1 and 4, then nums = [2,3]
- Remove numbers 2 and 3, then nums = []
There are no more pairs that sum up to 5, hence a total of 2 operations.```

Example 2:

```Input: nums = [3,1,3,4,3], k = 6
Output: 1
Explanation: Starting with nums = [3,1,3,4,3]:
- Remove the first two 3's, then nums = [1,4,3]
There are no more pairs that sum up to 6, hence a total of 1 operation.```

Constraints:

• `1 <= nums.length <= 105`
• `1 <= nums[i] <= 109`
• `1 <= k <= 109`

1679. Max Number of K-Sum Pairs
``````struct Solution;
use std::collections::HashMap;

impl Solution {
fn max_operations(nums: Vec<i32>, k: i32) -> i32 {
let mut count: HashMap<i32, usize> = HashMap::new();
for x in nums {
*count.entry(x).or_default() += 1;
}
let mut res = 0;
for &key in count.keys() {
let a = count.get(&key).unwrap_or(&0);
let b = count.get(&(k - key)).unwrap_or(&0);
res += a.min(b);
}
(res / 2) as i32
}
}

#[test]
fn test() {
let nums = vec![1, 2, 3, 4];
let k = 5;
let res = 2;
assert_eq!(Solution::max_operations(nums, k), res);
let nums = vec![3, 1, 3, 4, 3];
let k = 6;
let res = 1;
assert_eq!(Solution::max_operations(nums, k), res);
}
``````