Optimal Utilization

bool cmp( const vector<int>& v1,
               const vector<int>& v2 ) {
 return v1[1] < v2[1];
}
void fun (vector < vector <int> > &a, vector < vector <int> > &b, vector < vector <int> > &res, int target)
{

    int dif = INT_MIN;

    int l = 0;
    int r = b.size()-1;

    while (l < a.size() && r >=0)
    {
        int a_c = a[l][1];
        int b_c =  b[r][1];
        int cur_dif = a_c + b_c - target ;
        if (cur_dif > dif && cur_dif <= 0)
        {
            res.clear();

            res.push_back({a[l][0],b[r][0]});
            dif = cur_dif;

        }
        else if (cur_dif == dif)
        {

            res.push_back( {a[l][0],b[r][0]});
        }

        if (cur_dif > 0 )
            r--;
        else l++;

    }
}

int main() {
   vector < vector <int> >a = {{1, 3}, {2, 5}, {3, 7}, {4,10}};
    vector <vector <int> >  b = {{1, 2}, {2, 3}, {3, 4}, {4,5}};
    int target = 10;
    vector < vector <int> > res;

    sort(a.begin(), a.end(),cmp);
    sort(b.begin(), b.end(),cmp);

    fun (a,b,res,target);

    for (int i = 0 ; i < res.size(); i ++)
    {
        for (int j= 0 ; j < 2 ;  j ++)
            cout <<res[i][j] <<" ";
        cout << endl;
    }
}