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.

Solution:

class Solution {
    public int maxOperations(int[] nums, int k) {
        // base case
        if (nums == null || nums.length < 2){
            return -1;
        }

        Arrays.sort(nums);

        int count = 0;
        int left = 0;
        int right = nums.length -1;
        while(left < right){
            int sum = nums[left] + nums[right];
            if (sum == k ){
                count++;
                left++;
                right--;
            }else if (sum < k){
                left++;
            }else{
                right--;
            }

        }

        return count;

    }
}

// TC: O(nlogn)

// SC: O(n) or O(logN)

/*
In Java, the Arrays.sort() is implemented as a variant of quicksort algorithm whose space complexity is O(log⁡N) 
*/