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花花酱 LeetCode 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

Solution 1: Frequency Map

For each x, check freq[x] and freq[k – x]. Note: there is a special case when x + x == k.

Time complexity: O(n)
Space complexity: O(n)

C++

Python3

Solution 2: Two Pointers

Sort the number, start from i = 0, j = n – 1, compare s = nums[i] + nums[j] with k and move i, j accordingly.

Time complexity: O(nlogn)
Space complexity: O(1)

C++

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