Ordered set

#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/assoc_container.hpp>

using namespace __gnu_pbds;

void solve() {
    using kth_set = __gnu_pbds::tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update>;
    
    kth_set s;
    s.insert(3); // {3}
    s.insert(1); // {1, 3};
    auto it = s.lower_bound(0); // 1
    cout << s.order_of_key(*it) << endl; // find({1, 3}, 1) == 0
    auto it2 = s.lower_bound(2); // 3
    cout << s.order_of_key(*it2) << endl; // find({1, 3}, 3) == 1
}

Souce 1 — Ordered set
Source 2 — Blog