1679. Max Number of K-Sum Pairs

https://leetcode.com/problems/max-number-of-k-sum-pairs/

Python

Prefix Sum

from collections import defaultdict


class Solution:
    def maxOperations(self, nums: List[int], k: int) -> int:
        if len(nums) < 2:
            return 0

        mapper = defaultdict(int)
        for i, num in enumerate(nums):
            mapper[num] += 1

        used = set()
        ans = 0
        for num, count in mapper.items():
            if num in used:
                continue

            remains = k - num
            if remains == num:
                ans += count // 2
                used.add(num)
            elif remains in mapper:
                ans += min(count, mapper[remains])
                used.add(remains)

        return ans