package hashMaps;

import java.util.HashMap;

import java.util.Scanner;

public class PairSumToZero {

private static int pairSumToZero(int[] arr) {

// Storing the element of array, and it's frequency in map

var map = new HashMap<Integer, Integer>();

for (int i : arr) {

if (map.containsKey(i)) map.put(i, map.get(i) + 1);

else map.put(i, 1);

}

// Traversing the array checking if the element is present in hashmap or not

int finalCount = 0;

for (int i : arr) {

if (map.containsKey(-i) && map.get(i) != 0) {

int times;

if (i == (-i)) { // true in case of 0

int occurrences = map.get(i);

times = (occurrences * (occurrences - 1)) / 2;

finalCount += times;

map.put(i, 0);

continue;

}

times = map.get(i) * map.get(-i);

finalCount += times;

map.put(i, 0);

map.put(-i, 0);

}

}

return finalCount;

}

public static void main(String[] args) {

var arr = takeArrayInput();

System.out.println(pairSumToZero(arr));

}

private static int[] takeArrayInput() {

Scanner scan = new Scanner(System.in);

System.out.print("Enter the size of the array: ");

int size = scan.nextInt();

int[] arr = new int[size];

System.out.print("Enter the elements of the array: ");

for (int i = 0; i < size; i++) arr[i] = scan.nextInt();

return arr;

}

}