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
SOLUTION:
from collections import Counter

class Solution:
    def maxOperations(self, nums: List[int], k: int) -> int:
        pairs = 0
        ctr = Counter(nums)
        for num in ctr:
            if 2 * num <= k and (k - num) in ctr:
                a = ctr[num]
                b = ctr[k - num]
                if 2 * num == k:
                    pairs += a // 2
                else:
                    pairs += min(a, b)
        return pairs

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