pbds::Tree example

// Problem: https://sirius0621.contest.codeforces.com/group/a7fj9QBnKu/contest/332231/problem/C
#include <bits/stdc++.h>
// tiom4eg's precompiler options
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// IO settings
#define fastIO ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0)
// Quick types
#define ll long long
#define ull unsigned long long
#define pii pair <int, int>
#define pll pair <ll, ll>
#define vi vector <int>
#define mi vector <vector <int> >
// Quick functions
#define endl "\n"
#define F first
#define S second
#define all(a) a.begin(), a.end()
#define pb push_back
#define fint(n) int n; cin >> n
#define fstr(s) string s; cin >> s
#define farr(a, n) int a[n]; for (int pog = 0; pog < n; ++pog) cin >> a[pog]
// Quick fors
#define FOR1(s, n) for (int i = s; i < n; ++i)
#define FOR2(s, n) for (int j = s; j < n; ++j)
#define FOR3(s, n) for (int k = s; k < n; ++k)
#define RFOR(n, s) for (int l = n; l >= s; --l)
#define lk(x) (x << 1)
#define rk(x) (x << 1) + 1
// Pragmas
#pragma GCC optimize("Ofast")
#pragma target("avx,fma")
#pragma comment(linker, "/stack:200000000")
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#define ordered_multiset tree <pii, null_type, less <pii>, rb_tree_tag, tree_order_statistics_node_update>
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using namespace std;
using namespace __gnu_pbds;

int main() {
    fastIO;
    map <int, pii> used;
    ordered_multiset ordset;
    fint(q);
    FOR1(0, q) {
        fstr(op);
        if (op == "insert") {
            fint(x);
            if (used.find(x) == used.end()) {
                ordset.insert({x, 1});
                used[x] = {2, 1};
            }
            else {
                ordset.insert({x, used[x].F});
                used[x] = {used[x].F + 1, used[x].S};
            }
        }
        else if (op == "erase") {
            fint(x);
            ordset.erase(ordset.find({x, used[x].S}));
            used[x] = {used[x].F, used[x].S + 1};
        }
        else if (op == "count") {
            fint(x);
            cout << used[x].F - used[x].S << endl;
        }
        else if (op == "kth") {
            fint(k);
            cout << (*ordset.find_by_order(k - 1)).F << endl;
        }
        else {
            fint(x);
            cout << ordset.order_of_key({x, used[x].S}) << endl;
        }
    }
}