Given a array and given a range[l, r], determine the number of elements in the array greater than equal to l and less than equal to r

#include<bits/stdc++.h>

#include<ext/pb_ds/assoc_container.hpp>

#include<ext/pb_ds/tree_policy.hpp>

#define order_set tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update>

#define endl "\n\n"

using namespace __gnu_pbds;

using namespace std;

int main()

{

    order_set os;

    int n, x, l, r;

    cout << "Enter Array Size :: ";

    cin >> n;

    cout << "Enter Array Element :: ";

    for(int i=1; i<=n; i++)

    {

        cin >> x;

        os.insert(x);

    }

    cout << "Enter left range and right range :: ";

    cin >> l >> r;

    cout << "\nNumber of Element between " << l << " and " << r << " in this array is :: ";

    cout << os.order_of_key(r+1) - os.order_of_key(l) << "\n";

    return 0;

}