8RW7ej - Online C++0x Compiler & Debugging Tool

#include<bits/stdc++.h>
#include <iostream>
using namespace std;

int countPair(vector<int> arr,int k){
    unordered_map<int,int> freq;
    int count=0;
    for(int i=0;i<arr.size();i++){
        int complement=k-arr[i];
        if(freq.find(complement) != freq.end()){
            count = count+freq[complement];
        }

        freq[arr[i]]++;

    }
    return count;

}

int main(){
    vector<int> arr={1, 5, 7, -1, 5};
    int k=6;
    cout<<"No. of Pairs: "<<countPair(arr,k);
}