Count_pairs_with_given_sum.cpp

/*
Problem Statement:
------------------
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)
*/

// Link --> https://practice.geeksforgeeks.org/problems/count-pairs-with-given-sum5022/1#

// Code:
class Solution
{   
public:
    int getPairsCount(int a[], int n, int k)
    {
        unordered_map <int , int> m;

        for(int i=0 ; i<n ; i++) 
            m[a[i]]++;
        
        int count = 0;
        for(int i=0 ; i<n ; i++) 
        {
            count += m[k - a[i]];
            
            if(k - a[i] == a[i])
                count--;
        }
        return count / 2;
    }
};