PairSumto0.java

// Pair Sum to 0
// Send Feedback
// 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
// Explanation
// (2,-2) , (-2,2) will result in 0 , so the answer for the above problem is 2.

import java.util.*;

public class Solution {
    public static int PairSum(int[] input, int size) {
        /*
         * Your class should be named Solution
         * Don't write main().
         * Don't read input, it is passed as function argument.
         * Return output and don't print it.
         * Taking input and printing output is handled automatically.
         */
        HashMap<Integer, Integer> map = new HashMap<>();
        int count = 0;
        for (int i : input) {
            int complement = -i;
            if (map.containsKey(complement)) {
                count += map.get(complement);
            }
            if (map.containsKey(i)) {
                map.put(i, map.get(i) + 1);
            } else {
                map.put(i, 1);
            }
        }
        return count;
    }
}