Max Number of K-Sum Pairs - Two Sum Approach

Array     C++     Hash Table     Medium     Sorting     Two Pointers    

Problem Statement:

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:

We use a HashMap based standard two sum approach.

 
class Solution {
public:
    int maxOperations(vector<int>& nums, int target) {
        unordered_map<int,int> comps;
        int ctr=0;
        for (int n: nums)
        {
            if (comps.count(n) && comps[n]>0)
            {
                ctr++;
                comps[n]--;
            } else comps[target-n]++;
        }
        return ctr;
    }
};
 

 
TC: O(n)
SC: O(n)