Pair sum to 0.cpp

/*
Pair Sum to 0
Given a random integer array A of size N. Find and print the count of pair of elements in the array which sum up to 0.

Note: Array A can contain duplicate elements as well.

Input format:
The first line of input contains an integer, that denotes the value of the size of the array. Let us denote it with the symbol N.
The following line contains N space separated integers, that denote the value of the elements of the array.

Output format :
The first and only line of output contains the count of pair of elements in the array which sum up to 0. 

Constraints :
0 <= N <= 10^4

Time Limit: 1 sec

Sample Input 1:
5
2 1 -2 2 3

Sample Output 1:
2
*/

#include<unordered_map>
int pairSum(int *arr, int n) {
	// Write your code here
    unordered_map<int, int> map;
    
    for(int i = 0; i < n; i++) {
        map[arr[i]]++;
    }
    
    int count = 0;
    
    for(int i = 0; i < n; i++) {
        if(arr[i] == 0 and map[arr[i]] > 1) {
            count += ((map[arr[i]]) *(map[arr[i]] - 1)) / 2;// n choose 2
            map[arr[i]] = 0;
        } else if(map[-arr[i]] and arr[i] != 0) {
            count += map[arr[i]] * map[-arr[i]];
            map[arr[i]] = 0;
            map[-arr[i]] = 0;
        }
    }
    
    return count;
}

// Time Complexity : O(n)
// Auxilary Space : O(n)