1679. Max Number of K-Sum Pairs

1. Description

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.

2. Example

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.

3. Constraints

• 1 <= nums.length <= $10^5$
• 1 <= nums[i] <= $10^9$
• 1 <= k <= $10^9$

4. Solutions

Hash Table

n = nums.size()
Time complexity: O(n)
Space complexity: O(n)

class Solution {
public:
    int maxOperations(const vector<int> &nums, int k) {
        unordered_map<int, int> num_count;
        int valid_options = 0;
        for (int i = 0; i < nums.size(); ++i) {
            if (nums[i] < k) {
                if (num_count.contains(k - nums[i]) && num_count[k - nums[i]] > 0) {
                    --num_count[k - nums[i]];
                    ++valid_options;
                } else {
                    ++num_count[nums[i]];
                }
            }
        }

        return valid_options;
    }
};