Sum of Pairs: Given an array of integers and a number K, check if there exist a pair of indices i,j s.t. a[i] + a[j] = K and i!=j.

sum_pairs.cpp

#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;

string sum_pair(vector<int> &ar, const int k) {
    sort(ar.begin(), ar.end());
    for(int i = 0, j = ar.size()-1, sum = ar[i] + ar[j]; i < j; sum = ar[i] + ar[j]) {
        if(sum == k) return "True";
        else if(sum < k) ++i;
        else --j;
    }
    return "False";
}

int main() {
    int t;  cin >> t;
    while(t--) {
        int n, k;  cin >> n >> k;
        
        vector<int> ar(n);
        for(int i = 0; i < n; ++i)
            cin >> ar[i];
        
        cout << sum_pair(ar, k) << endl;
    }
    return 0;
}