Count pairs with given sum.cpp

/*
Count pairs with given sum
==========================

Given an array of N integers, and an integer K, find the number of pairs of elements in the array whose sum is equal to K.

Example 1:
Input:
N = 4, K = 6
arr[] = {1, 5, 7, 1}
Output: 2
Explanation: 
arr[0] + arr[1] = 1 + 5 = 6 
and arr[1] + arr[3] = 5 + 1 = 6.

Example 2:
Input:
N = 4, X = 2
arr[] = {1, 1, 1, 1}
Output: 6
Explanation: 
Each 1 will produce sum 2 with any 1.
Your Task:
You don't need to read input or print anything. Your task is to complete the function getPairsCount() which takes arr[], n and k as input parameters and returns the number of pairs that have sum K.

Expected Time Complexity: O(N)
Expected Auxiliary Space: O(N)

Constraints:
1 <= N <= 105
1 <= K <= 108
1 <= Arr[i] <= 106
*/

int getPairsCount(int arr[], int n, int k)
{
  unordered_map<int, int> freq;
  for (int i = 0; i < n; ++i)
    freq[arr[i]]++;
  int ans = 0;
  for (int i = 0; i < n; ++i)
  {
    ans += freq[k - arr[i]];
    if (k == 2 * arr[i])
      ans--;
  }
  return ans / 2;
}