Pairsumtozero.cpp

/*

				Name: Mehul Chaturvedi
				IIT-Guwahati

*/

/*
			PROBLEM STATEMENT
Given a random integer array A of size N. Find and print the pair of elements in the array which sum to 0.
Array A can contain duplicate elements.
While printing a pair, print the smaller element first.
That is, if a valid pair is (6, -6) print "-6 6". There is no constraint that out of 5 pairs which have to be printed in 1st line. 
You can print pairs in any order, just be careful about the order of elements in a pair.

*/

#include <bits/stdc++.h>

using namespace std;

void PairSum(int *input, int n) {	 
	unordered_map<int, int> m1;
	for (int i = 0; i < n; ++i)
	{
		if(m1[0-input[i]]==0){
			m1[input[i]]++;
            
		}else{
            m1[input[i]]++;
		}
	}
    
    unordered_map<int, int>::iterator it=m1.begin();
    while(it!=m1.end()){
        int total = 0;
        int left = it->second;
        int right = m1[-it->first];
        total = left*right;
        while(total>0){
            cout << min(it->first, -it->first)<<" "<< max(it->first, -it->first) << '\n';
            total--;
        }
        m1[it->first]=0;
        m1[-it->first]=0;
        it++;
        
    }
     	
}

int main( int argc , char ** argv )
{
	ios_base::sync_with_stdio(false) ; 
	cin.tie(NULL) ; 
	
	int n;
    int arr[100000];
    cin>>n;
    for(int i=0; i<n; i++)
    {
        cin>>arr[i];
    }
    PairSum(arr, n);
    return 0;



	return 0 ; 



}