#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
typedef tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> Tree;

Tree s;
s.insert(5);
s.erase(5);
s.find(5);
s.lower_bound(5);
s.count(5);
s.order_by_key(5); // номер элемента
s.find_by_order(1); // k-ая порядковая статистика


https://codeforces.com/blog/entry/11080?locale=ru