1679. Max Number of K-Sum Pairs

Problem

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

Solution (Java)

class Solution {
    public int maxOperations(int[] nums, int k) {
        Arrays.sort(nums);
        int start = 0;
        int end = nums.length - 1;
        int count = 0;
        while (start < end) {
            int sum = nums[start] + nums[end];
            if (sum == k) {
                count++;
                start++;
                end--;
            } else if (sum < k) {
                start++;
            } else {
                end--;
            }
        }
        return count;
    }
}

Explain:

nope.

Complexity:

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