Code : 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) {
  unordered_map<int, int> map;
  int mycount = 0;
  for (int i = 0; i < n; i++) {
    map[arr[i]]++;
  }
  mycount += map[0] * (map[0] - 1) / 2;
  map[0] = 0;
  for (int i = 0; i < n; i++) {
    if (map[-arr[i]] > 0) {
      mycount += map[arr[i]] * map[-arr[i]];
      map[arr[i]] = 0;
      map[-arr[i]] = 0;
    }
  }
  return mycount;
}

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