1679. Max Number of K-Sum Pairs.md

1679. Max Number of K-Sum Pairs

https://leetcode.com/problems/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 <= 10**5
• 1 <= nums[i] <= 10**9
• 1 <= k <= 10**9

代码

• HashMap / dictionary

class Solution:
    def maxOperations(self, nums: List[int], k: int) -> int:
        f = defaultdict(int)
        for x in nums:
            f[x] += 1
        
        ans = 0
        for x in nums:
            if k - x not in f:
                continue
            if f[x] == 0 or f[k - x] == 0:
                continue
            elif x == k - x and f[x] == 1:
                continue
            else:
                f[x] -= 1
                f[k - x] -= 1
                ans += 1
        return ans