Rainy2k
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ordered_set

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可以在set中找到小于某个数的元素的个数

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#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 ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;

int main(){
	ordered_set<int>  s;
	s.insert(1);
    s.insert(2);
    s.insert(3);
    s.insert(4);
    cout << s.order_of_key(1) << endl; // the number of elements in the s less than 1
    cout << s.order_of_key(2) << endl; // the number of elements in the s less than 2
    cout << s.order_of_key(3) << endl; // the number of elements in the s less than 3
    cout << s.order_of_key(4) << endl; // the number of elements in the s less than 4
	cout << *s.find_by_order(0) << endl; // print the 0-th smallest number in s(0-based)
    auto it = s.find_by_order(1);
    cout << *it << endl;//3
}