Ordered Set

#include<bits/stdc++.h>
#include<ext/pb_ds/assoc_container.hpp>
#include<ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
using namespace std;

template <typename T> using o_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
template <typename T, typename R> using o_map = tree<T, R, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
int main() {
  int i, j, k, n, m;
  o_set<int>se;
  se.insert(1);
  se.insert(2);
  cout << *se.find_by_order(0) << endl; ///k th element
  cout << se.order_of_key(2) << endl; ///number of elements less than k
  o_map<int, int>mp;
  mp.insert({1, 10});
  mp.insert({2, 20});
  cout << mp.find_by_order(0)->second << endl; ///k th element
  cout << mp.order_of_key(2) << endl; ///number of first elements less than k
  return 0;
}