pairSum.cpp

//Check if there exists two elements in an array such that their is equal to given k
#include<iostream>
#include<climits>
using namespace std;

bool pairSum(int arr[], int n, int k){
    for(int i=0; i<n-1; i++){
        for(int j=i+1; j<n; j++){
            if(arr[i]+arr[j] == k){
                cout<<i<<" "<<j<<endl;
                return true;
            }
        }
    }
    return false;
}

int main(){

    int arr[] = {2, 4, 7, 11, 14, 16, 20, 21};
    int k = 31;

    cout<<pairSum(arr,8,k)<<endl;
}

//Check if there exists two elements in an array such that their is equal to given k, time complexity O(n)
//for this method to be applied array must be sorted
#include<iostream>
#include<climits>
using namespace std;

bool pairSum(int arr[], int n, int k){
    int low = 0, high = n - 1;
    while(low < high){
        if(arr[low]+arr[high]==k){
            cout<<low+1<<" "<<high+1<<endl;
            return true;
            break;
        }
        else if(arr[low]+arr[high]<k){
            low++;
        }
        else{
            high--;
        }
    }
    return false;
}

int main(){
    int n;
    cin >> n;
    int a[n];

    for (int i = 0; i < n; i++){
        cin >> a[i];
    }

    int k;
    cin >> k;

    cout<<pairSum(a,n,k)<<endl;
}