Ordered Set and GNU C++ PBDS

#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;

tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> x;
tree<int, null_type, greater<int>, rb_tree_tag, tree_order_statistics_node_update> y;
tree<int, null_type, less_equal<int>, rb_tree_tag, tree_order_statistics_node_update> p;
tree<int, null_type, greater_equal<int>, rb_tree_tag, tree_order_statistics_node_update> q;


void solve() {
	int k = 5;
	x.insert(k);
	// Number of items strictly smaller than k
	x.order_of_key(k);
	// K-th element in a set (counting from zero)
	x.find_by_order(k);
}