Ordered Multiset
(ds/bst/ordered_multiset.hpp)

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• Last update: 2026-07-13 03:01:19+09:00
• Include: #include "ds/bst/ordered_multiset.hpp"

Overview

OrderedMultiset is a weight-balanced binary search tree for multisets. It stores equal keys as one node with a multiplicity, so it supports standard multiset operations plus order-statistics queries such as k-th element and rank. Nodes come from a recyclable block arena, avoiding general-purpose per-node allocation.

The arena is shared by each OrderedMultiset<T, Compare> specialization. Erased or destroyed nodes are reused, while the arena’s peak capacity is retained until program exit.

split(key) consumes a tree and partitions it into keys < key and keys >= key. merge(other) consumes two trees whose key ranges are strictly ordered. Equal keys therefore cannot occur across the merge boundary.

Pointers returned by bound and predecessor/successor methods remain valid until the multiset is modified.

Template Parameters

• T: The key type.
• Compare: Ordering predicate. Defaults to std::less<T>.

Trees passed to merge must use equivalent comparator state.

Constructors

• OrderedMultiset() Constructs an empty multiset. ($O(1)$)

• OrderedMultiset(Compare compare) Constructs an empty multiset with a custom comparator. ($O(1)$)

• OrderedMultiset(std::initializer_list<T> init) Constructs a multiset from an initializer list. ($O(N \log N)$)

• OrderedMultiset(Iterator first, Iterator last) Constructs a multiset from a range. ($O(N \log N)$)

Methods

Method	Description	Complexity
int size() const	Returns the total number of elements, including duplicates.	$O(1)$
int unique_size() const	Returns the number of distinct keys.	$O(1)$
bool empty() const	Returns whether the multiset is empty.	$O(1)$
void clear()	Removes all elements and returns their nodes to the arena.	$O(N)$
void insert(T key, int multiplicity = 1)	Inserts multiplicity copies of key.	$O(\log N)$
bool erase_one(const T& key)	Removes one copy of key; returns whether an element was removed.	$O(\log N)$
bool erase(const T& key)	Alias for erase_one(key).	$O(\log N)$
int erase_all(const T& key)	Removes all copies of key and returns the number removed.	$O(\log N)$
bool contains(const T& key) const	Returns whether key exists.	$O(\log N)$
int count(const T& key) const	Returns the multiplicity of key.	$O(\log N)$
const T* find_by_order(int k) const	Returns a pointer to the 0-indexed k-th smallest element. Requires 0 <= k < size().	$O(\log N)$
T kth(int k) const	Returns the 0-indexed k-th smallest element by value. Requires 0 <= k < size().	$O(\log N)$
int order_of_key(const T& key) const	Returns the number of elements strictly less than key.	$O(\log N)$
int count_less(const T& key) const	Alias for order_of_key(key).	$O(\log N)$
int count_less_equal(const T& key) const	Returns the number of elements less than or equal to key.	$O(\log N)$
int count_greater(const T& key) const	Returns the number of elements strictly greater than key.	$O(\log N)$
int count_greater_equal(const T& key) const	Returns the number of elements greater than or equal to key.	$O(\log N)$
const T* lower_bound(const T& key) const, const T* min_ge(const T& key) const	Returns the smallest element greater than or equal to key, or nullptr.	$O(\log N)$
const T* upper_bound(const T& key) const, const T* min_gt(const T& key) const	Returns the smallest element strictly greater than key, or nullptr.	$O(\log N)$
const T* max_le(const T& key) const	Returns the largest element less than or equal to key, or nullptr.	$O(\log N)$
const T* max_lt(const T& key) const	Returns the largest element strictly less than key, or nullptr.	$O(\log N)$
const T* min() const, const T* max() const	Returns the minimum or maximum element, or nullptr if the multiset is empty.	$O(\log N)$
std::pair<OrderedMultiset, OrderedMultiset> split(const T& key) &&	Consumes the multiset and returns {less, greater_equal}.	$O(\log N)$
OrderedMultiset merge(OrderedMultiset other) &&	Consumes both multisets and returns their union. Requires every key in *this to be smaller than every key in other.	$O(\log(N + M))$
std::vector<T> to_vector() const	Returns all elements in sorted order, including duplicates.	$O(N)$

Example

#include "ds/bst/ordered_multiset.hpp"

#include <iostream>
#include <utility>

int main() {
    m1une::ds::OrderedMultiset<int> ms = {3, 1, 3, 5};

    ms.insert(2);
    ms.erase_one(3);

    auto [small, large] = std::move(ms).split(3);
    ms = std::move(small).merge(std::move(large));

    std::cout << ms.kth(2) << "\n";           // 3
    std::cout << ms.order_of_key(4) << "\n";  // 3

    if (auto p = ms.max_le(4)) {
        std::cout << *p << "\n";              // 3
    }
}

Required by

• Rollback Ordered Multiset (ds/bst/rollback_ordered_multiset.hpp)

Verified with

• verify/ds/bst/ordered_multiset.test.cpp
• verify/ds/rollback_counterparts.test.cpp

Code

#ifndef M1UNE_ORDERED_MULTISET_HPP
#define M1UNE_ORDERED_MULTISET_HPP 1

#include <cassert>
#include <functional>
#include <initializer_list>
#include <memory>
#include <utility>
#include <vector>

namespace m1une {
namespace ds {

template <typename T, typename Compare = std::less<T>>
struct OrderedMultiset {
   private:
    struct Node {
        T key;
        int count;
        int size;
        int distinct_size;
        Node* l;
        Node* r;

        Node(T value, int multiplicity)
            : key(std::move(value)),
              count(multiplicity),
              size(multiplicity),
              distinct_size(1),
              l(nullptr),
              r(nullptr) {}
    };

    static constexpr int pool_block_size = 1 << 14;

    struct Pool {
        std::vector<std::vector<Node>> blocks;
        std::vector<Node*> free_nodes;

        template <class... Args>
        Node* emplace(Args&&... args) {
            if (!free_nodes.empty()) {
                Node* result = free_nodes.back();
                free_nodes.pop_back();
                std::destroy_at(result);
                std::construct_at(result, std::forward<Args>(args)...);
                return result;
            }
            if (blocks.empty() || int(blocks.back().size()) == pool_block_size) {
                blocks.emplace_back();
                blocks.back().reserve(pool_block_size);
            }
            blocks.back().emplace_back(std::forward<Args>(args)...);
            return &blocks.back().back();
        }

        void recycle(Node* node) {
            free_nodes.push_back(node);
        }
    };

    inline static Pool pool;

    Node* root;
    Compare comp;

    static int subtree_size(const Node* t) {
        return t == nullptr ? 0 : t->size;
    }

    static int subtree_distinct_size(const Node* t) {
        return t == nullptr ? 0 : t->distinct_size;
    }

    bool equal(const T& a, const T& b) const {
        return !comp(a, b) && !comp(b, a);
    }

    Node* new_node(T key, int multiplicity) {
        return pool.emplace(std::move(key), multiplicity);
    }

    static void update(Node* t) {
        t->size = t->count + subtree_size(t->l) + subtree_size(t->r);
        t->distinct_size = 1 + subtree_distinct_size(t->l) + subtree_distinct_size(t->r);
    }

    static Node* rotate_right(Node* t) {
        Node* s = t->l;
        t->l = s->r;
        s->r = t;
        update(t);
        update(s);
        return s;
    }

    static Node* rotate_left(Node* t) {
        Node* s = t->r;
        t->r = s->l;
        s->l = t;
        update(t);
        update(s);
        return s;
    }

    static Node* balance(Node* t) {
        if (t == nullptr) return nullptr;
        const int left_size = subtree_distinct_size(t->l);
        const int right_size = subtree_distinct_size(t->r);
        if (left_size + right_size > 1 && left_size > 3LL * right_size) {
            if (subtree_distinct_size(t->l->r) >= 2LL * subtree_distinct_size(t->l->l)) {
                t->l = rotate_left(t->l);
            }
            return rotate_right(t);
        }
        if (left_size + right_size > 1 && right_size > 3LL * left_size) {
            if (subtree_distinct_size(t->r->l) >= 2LL * subtree_distinct_size(t->r->r)) {
                t->r = rotate_right(t->r);
            }
            return rotate_left(t);
        }
        update(t);
        return t;
    }

    static Node* join_with_root(Node* l, Node* middle, Node* r) {
        const int left_size = subtree_distinct_size(l);
        const int right_size = subtree_distinct_size(r);
        if (left_size > 3LL * (right_size + 1)) {
            l->r = join_with_root(l->r, middle, r);
            return balance(l);
        }
        if (right_size > 3LL * (left_size + 1)) {
            r->l = join_with_root(l, middle, r->l);
            return balance(r);
        }
        middle->l = l;
        middle->r = r;
        return balance(middle);
    }

    static Node* detach_max(Node* t, Node*& maximum) {
        if (t->r == nullptr) {
            maximum = t;
            return t->l;
        }
        t->r = detach_max(t->r, maximum);
        return balance(t);
    }

    static Node* merge_nodes(Node* l, Node* r) {
        if (l == nullptr || r == nullptr) return l == nullptr ? r : l;
        Node* middle;
        l = detach_max(l, middle);
        return join_with_root(l, middle, r);
    }

    std::pair<Node*, Node*> split_nodes(Node* t, const T& key) {
        if (t == nullptr) return {nullptr, nullptr};
        Node* left = t->l;
        Node* right = t->r;
        t->l = nullptr;
        t->r = nullptr;
        if (comp(t->key, key)) {
            auto [l, r] = split_nodes(right, key);
            return {join_with_root(left, t, l), r};
        }
        auto [l, r] = split_nodes(left, key);
        return {l, join_with_root(r, t, right)};
    }

    Node* insert_impl(Node* t, T& key, int multiplicity, bool& new_key) {
        if (t == nullptr) {
            new_key = true;
            return new_node(std::move(key), multiplicity);
        }
        if (comp(key, t->key)) {
            t->l = insert_impl(t->l, key, multiplicity, new_key);
        } else if (comp(t->key, key)) {
            t->r = insert_impl(t->r, key, multiplicity, new_key);
        } else {
            t->count += multiplicity;
            t->size += multiplicity;
            new_key = false;
            return t;
        }
        if (!new_key) {
            t->size += multiplicity;
            return t;
        }
        return balance(t);
    }

    Node* erase_impl(Node* t, const T& key, bool erase_all,
                     int& erased, bool& removed_key) {
        if (t == nullptr) return nullptr;
        if (comp(key, t->key)) {
            t->l = erase_impl(t->l, key, erase_all, erased, removed_key);
        } else if (comp(t->key, key)) {
            t->r = erase_impl(t->r, key, erase_all, erased, removed_key);
        } else if (!erase_all && t->count > 1) {
            --t->count;
            --t->size;
            erased = 1;
            removed_key = false;
            return t;
        } else {
            erased = t->count;
            removed_key = true;
            Node* l = t->l;
            Node* r = t->r;
            pool.recycle(t);
            return merge_nodes(l, r);
        }
        if (erased == 0) return t;
        if (!removed_key) {
            t->size -= erased;
            return t;
        }
        return balance(t);
    }

    static const T* kth_impl(const Node* t, int k) {
        while (t != nullptr) {
            const int left_size = subtree_size(t->l);
            if (k < left_size) {
                t = t->l;
            } else if (k < left_size + t->count) {
                return &t->key;
            } else {
                k -= left_size + t->count;
                t = t->r;
            }
        }
        return nullptr;
    }

    int count_impl(const Node* t, const T& key) const {
        while (t != nullptr) {
            if (comp(key, t->key)) {
                t = t->l;
            } else if (comp(t->key, key)) {
                t = t->r;
            } else {
                return t->count;
            }
        }
        return 0;
    }

    int order_of_key_impl(const Node* t, const T& key, bool upper) const {
        int result = 0;
        while (t != nullptr) {
            const bool take = upper ? !comp(key, t->key) : comp(t->key, key);
            if (take) {
                result += subtree_size(t->l) + t->count;
                t = t->r;
            } else {
                t = t->l;
            }
        }
        return result;
    }

    const T* lower_bound_impl(const Node* t, const T& key, bool strict) const {
        const T* result = nullptr;
        while (t != nullptr) {
            const bool candidate = strict ? comp(key, t->key) : !comp(t->key, key);
            if (candidate) {
                result = &t->key;
                t = t->l;
            } else {
                t = t->r;
            }
        }
        return result;
    }

    const T* max_less_impl(const Node* t, const T& key, bool strict) const {
        const T* result = nullptr;
        while (t != nullptr) {
            const bool candidate = strict ? comp(t->key, key) : !comp(key, t->key);
            if (candidate) {
                result = &t->key;
                t = t->r;
            } else {
                t = t->l;
            }
        }
        return result;
    }

    static void dump_impl(const Node* t, std::vector<T>& result) {
        if (t == nullptr) return;
        dump_impl(t->l, result);
        for (int i = 0; i < t->count; ++i) result.push_back(t->key);
        dump_impl(t->r, result);
    }

    static void recycle_impl(Node* t) {
        if (t == nullptr) return;
        recycle_impl(t->l);
        recycle_impl(t->r);
        pool.recycle(t);
    }

    Node* clone_impl(const Node* t) {
        if (t == nullptr) return nullptr;
        Node* result = new_node(t->key, t->count);
        result->l = clone_impl(t->l);
        result->r = clone_impl(t->r);
        update(result);
        return result;
    }

    OrderedMultiset(Node* node, Compare compare) : root(node), comp(std::move(compare)) {}

   public:
    explicit OrderedMultiset(Compare compare) : root(nullptr), comp(std::move(compare)) {}
    OrderedMultiset() : OrderedMultiset(Compare()) {}

    OrderedMultiset(std::initializer_list<T> init, Compare compare = Compare())
        : OrderedMultiset(std::move(compare)) {
        for (const T& x : init) insert(x);
    }

    template <typename Iterator>
    OrderedMultiset(Iterator first, Iterator last, Compare compare = Compare())
        : OrderedMultiset(std::move(compare)) {
        while (first != last) insert(*first++);
    }

    OrderedMultiset(const OrderedMultiset& other) : root(nullptr), comp(other.comp) {
        root = clone_impl(other.root);
    }

    OrderedMultiset(OrderedMultiset&& other) noexcept
        : root(std::exchange(other.root, nullptr)), comp(std::move(other.comp)) {}

    ~OrderedMultiset() {
        recycle_impl(root);
    }

    OrderedMultiset& operator=(OrderedMultiset other) {
        swap(other);
        return *this;
    }

    void swap(OrderedMultiset& other) noexcept {
        using std::swap;
        swap(root, other.root);
        swap(comp, other.comp);
    }

    int size() const { return subtree_size(root); }
    int unique_size() const { return subtree_distinct_size(root); }
    bool empty() const { return root == nullptr; }

    void clear() {
        recycle_impl(root);
        root = nullptr;
    }

    void insert(T key, int multiplicity = 1) {
        assert(multiplicity > 0);
        bool new_key = false;
        root = insert_impl(root, key, multiplicity, new_key);
    }

    bool erase_one(const T& key) {
        int erased = 0;
        bool removed_key = false;
        root = erase_impl(root, key, false, erased, removed_key);
        return erased != 0;
    }

    bool erase(const T& key) { return erase_one(key); }

    int erase_all(const T& key) {
        int erased = 0;
        bool removed_key = false;
        root = erase_impl(root, key, true, erased, removed_key);
        return erased;
    }

    bool contains(const T& key) const { return count(key) > 0; }
    int count(const T& key) const { return count_impl(root, key); }

    const T* find_by_order(int k) const {
        assert(0 <= k && k < size());
        return kth_impl(root, k);
    }

    T kth(int k) const { return *find_by_order(k); }
    int order_of_key(const T& key) const { return order_of_key_impl(root, key, false); }
    int count_less(const T& key) const { return order_of_key(key); }
    int count_less_equal(const T& key) const { return order_of_key_impl(root, key, true); }
    int count_greater(const T& key) const { return size() - count_less_equal(key); }
    int count_greater_equal(const T& key) const { return size() - count_less(key); }
    const T* lower_bound(const T& key) const { return lower_bound_impl(root, key, false); }
    const T* upper_bound(const T& key) const { return lower_bound_impl(root, key, true); }
    const T* min_ge(const T& key) const { return lower_bound(key); }
    const T* min_gt(const T& key) const { return upper_bound(key); }
    const T* max_le(const T& key) const { return max_less_impl(root, key, false); }
    const T* max_lt(const T& key) const { return max_less_impl(root, key, true); }
    const T* min() const { return empty() ? nullptr : kth_impl(root, 0); }
    const T* max() const { return empty() ? nullptr : kth_impl(root, size() - 1); }

    std::pair<OrderedMultiset, OrderedMultiset> split(const T& key) && {
        auto [l, r] = split_nodes(root, key);
        root = nullptr;
        return {OrderedMultiset(l, comp), OrderedMultiset(r, std::move(comp))};
    }

    OrderedMultiset merge(OrderedMultiset other) && {
        assert(empty() || other.empty() || comp(*max(), *other.min()));
        root = merge_nodes(root, other.root);
        other.root = nullptr;
        return std::move(*this);
    }

    std::vector<T> to_vector() const {
        std::vector<T> result;
        result.reserve(size());
        dump_impl(root, result);
        return result;
    }
};

}  // namespace ds
}  // namespace m1une

#endif  // M1UNE_ORDERED_MULTISET_HPP

#line 1 "ds/bst/ordered_multiset.hpp"



#include <cassert>
#include <functional>
#include <initializer_list>
#include <memory>
#include <utility>
#include <vector>

namespace m1une {
namespace ds {

template <typename T, typename Compare = std::less<T>>
struct OrderedMultiset {
   private:
    struct Node {
        T key;
        int count;
        int size;
        int distinct_size;
        Node* l;
        Node* r;

        Node(T value, int multiplicity)
            : key(std::move(value)),
              count(multiplicity),
              size(multiplicity),
              distinct_size(1),
              l(nullptr),
              r(nullptr) {}
    };

    static constexpr int pool_block_size = 1 << 14;

    struct Pool {
        std::vector<std::vector<Node>> blocks;
        std::vector<Node*> free_nodes;

        template <class... Args>
        Node* emplace(Args&&... args) {
            if (!free_nodes.empty()) {
                Node* result = free_nodes.back();
                free_nodes.pop_back();
                std::destroy_at(result);
                std::construct_at(result, std::forward<Args>(args)...);
                return result;
            }
            if (blocks.empty() || int(blocks.back().size()) == pool_block_size) {
                blocks.emplace_back();
                blocks.back().reserve(pool_block_size);
            }
            blocks.back().emplace_back(std::forward<Args>(args)...);
            return &blocks.back().back();
        }

        void recycle(Node* node) {
            free_nodes.push_back(node);
        }
    };

    inline static Pool pool;

    Node* root;
    Compare comp;

    static int subtree_size(const Node* t) {
        return t == nullptr ? 0 : t->size;
    }

    static int subtree_distinct_size(const Node* t) {
        return t == nullptr ? 0 : t->distinct_size;
    }

    bool equal(const T& a, const T& b) const {
        return !comp(a, b) && !comp(b, a);
    }

    Node* new_node(T key, int multiplicity) {
        return pool.emplace(std::move(key), multiplicity);
    }

    static void update(Node* t) {
        t->size = t->count + subtree_size(t->l) + subtree_size(t->r);
        t->distinct_size = 1 + subtree_distinct_size(t->l) + subtree_distinct_size(t->r);
    }

    static Node* rotate_right(Node* t) {
        Node* s = t->l;
        t->l = s->r;
        s->r = t;
        update(t);
        update(s);
        return s;
    }

    static Node* rotate_left(Node* t) {
        Node* s = t->r;
        t->r = s->l;
        s->l = t;
        update(t);
        update(s);
        return s;
    }

    static Node* balance(Node* t) {
        if (t == nullptr) return nullptr;
        const int left_size = subtree_distinct_size(t->l);
        const int right_size = subtree_distinct_size(t->r);
        if (left_size + right_size > 1 && left_size > 3LL * right_size) {
            if (subtree_distinct_size(t->l->r) >= 2LL * subtree_distinct_size(t->l->l)) {
                t->l = rotate_left(t->l);
            }
            return rotate_right(t);
        }
        if (left_size + right_size > 1 && right_size > 3LL * left_size) {
            if (subtree_distinct_size(t->r->l) >= 2LL * subtree_distinct_size(t->r->r)) {
                t->r = rotate_right(t->r);
            }
            return rotate_left(t);
        }
        update(t);
        return t;
    }

    static Node* join_with_root(Node* l, Node* middle, Node* r) {
        const int left_size = subtree_distinct_size(l);
        const int right_size = subtree_distinct_size(r);
        if (left_size > 3LL * (right_size + 1)) {
            l->r = join_with_root(l->r, middle, r);
            return balance(l);
        }
        if (right_size > 3LL * (left_size + 1)) {
            r->l = join_with_root(l, middle, r->l);
            return balance(r);
        }
        middle->l = l;
        middle->r = r;
        return balance(middle);
    }

    static Node* detach_max(Node* t, Node*& maximum) {
        if (t->r == nullptr) {
            maximum = t;
            return t->l;
        }
        t->r = detach_max(t->r, maximum);
        return balance(t);
    }

    static Node* merge_nodes(Node* l, Node* r) {
        if (l == nullptr || r == nullptr) return l == nullptr ? r : l;
        Node* middle;
        l = detach_max(l, middle);
        return join_with_root(l, middle, r);
    }

    std::pair<Node*, Node*> split_nodes(Node* t, const T& key) {
        if (t == nullptr) return {nullptr, nullptr};
        Node* left = t->l;
        Node* right = t->r;
        t->l = nullptr;
        t->r = nullptr;
        if (comp(t->key, key)) {
            auto [l, r] = split_nodes(right, key);
            return {join_with_root(left, t, l), r};
        }
        auto [l, r] = split_nodes(left, key);
        return {l, join_with_root(r, t, right)};
    }

    Node* insert_impl(Node* t, T& key, int multiplicity, bool& new_key) {
        if (t == nullptr) {
            new_key = true;
            return new_node(std::move(key), multiplicity);
        }
        if (comp(key, t->key)) {
            t->l = insert_impl(t->l, key, multiplicity, new_key);
        } else if (comp(t->key, key)) {
            t->r = insert_impl(t->r, key, multiplicity, new_key);
        } else {
            t->count += multiplicity;
            t->size += multiplicity;
            new_key = false;
            return t;
        }
        if (!new_key) {
            t->size += multiplicity;
            return t;
        }
        return balance(t);
    }

    Node* erase_impl(Node* t, const T& key, bool erase_all,
                     int& erased, bool& removed_key) {
        if (t == nullptr) return nullptr;
        if (comp(key, t->key)) {
            t->l = erase_impl(t->l, key, erase_all, erased, removed_key);
        } else if (comp(t->key, key)) {
            t->r = erase_impl(t->r, key, erase_all, erased, removed_key);
        } else if (!erase_all && t->count > 1) {
            --t->count;
            --t->size;
            erased = 1;
            removed_key = false;
            return t;
        } else {
            erased = t->count;
            removed_key = true;
            Node* l = t->l;
            Node* r = t->r;
            pool.recycle(t);
            return merge_nodes(l, r);
        }
        if (erased == 0) return t;
        if (!removed_key) {
            t->size -= erased;
            return t;
        }
        return balance(t);
    }

    static const T* kth_impl(const Node* t, int k) {
        while (t != nullptr) {
            const int left_size = subtree_size(t->l);
            if (k < left_size) {
                t = t->l;
            } else if (k < left_size + t->count) {
                return &t->key;
            } else {
                k -= left_size + t->count;
                t = t->r;
            }
        }
        return nullptr;
    }

    int count_impl(const Node* t, const T& key) const {
        while (t != nullptr) {
            if (comp(key, t->key)) {
                t = t->l;
            } else if (comp(t->key, key)) {
                t = t->r;
            } else {
                return t->count;
            }
        }
        return 0;
    }

    int order_of_key_impl(const Node* t, const T& key, bool upper) const {
        int result = 0;
        while (t != nullptr) {
            const bool take = upper ? !comp(key, t->key) : comp(t->key, key);
            if (take) {
                result += subtree_size(t->l) + t->count;
                t = t->r;
            } else {
                t = t->l;
            }
        }
        return result;
    }

    const T* lower_bound_impl(const Node* t, const T& key, bool strict) const {
        const T* result = nullptr;
        while (t != nullptr) {
            const bool candidate = strict ? comp(key, t->key) : !comp(t->key, key);
            if (candidate) {
                result = &t->key;
                t = t->l;
            } else {
                t = t->r;
            }
        }
        return result;
    }

    const T* max_less_impl(const Node* t, const T& key, bool strict) const {
        const T* result = nullptr;
        while (t != nullptr) {
            const bool candidate = strict ? comp(t->key, key) : !comp(key, t->key);
            if (candidate) {
                result = &t->key;
                t = t->r;
            } else {
                t = t->l;
            }
        }
        return result;
    }

    static void dump_impl(const Node* t, std::vector<T>& result) {
        if (t == nullptr) return;
        dump_impl(t->l, result);
        for (int i = 0; i < t->count; ++i) result.push_back(t->key);
        dump_impl(t->r, result);
    }

    static void recycle_impl(Node* t) {
        if (t == nullptr) return;
        recycle_impl(t->l);
        recycle_impl(t->r);
        pool.recycle(t);
    }

    Node* clone_impl(const Node* t) {
        if (t == nullptr) return nullptr;
        Node* result = new_node(t->key, t->count);
        result->l = clone_impl(t->l);
        result->r = clone_impl(t->r);
        update(result);
        return result;
    }

    OrderedMultiset(Node* node, Compare compare) : root(node), comp(std::move(compare)) {}

   public:
    explicit OrderedMultiset(Compare compare) : root(nullptr), comp(std::move(compare)) {}
    OrderedMultiset() : OrderedMultiset(Compare()) {}

    OrderedMultiset(std::initializer_list<T> init, Compare compare = Compare())
        : OrderedMultiset(std::move(compare)) {
        for (const T& x : init) insert(x);
    }

    template <typename Iterator>
    OrderedMultiset(Iterator first, Iterator last, Compare compare = Compare())
        : OrderedMultiset(std::move(compare)) {
        while (first != last) insert(*first++);
    }

    OrderedMultiset(const OrderedMultiset& other) : root(nullptr), comp(other.comp) {
        root = clone_impl(other.root);
    }

    OrderedMultiset(OrderedMultiset&& other) noexcept
        : root(std::exchange(other.root, nullptr)), comp(std::move(other.comp)) {}

    ~OrderedMultiset() {
        recycle_impl(root);
    }

    OrderedMultiset& operator=(OrderedMultiset other) {
        swap(other);
        return *this;
    }

    void swap(OrderedMultiset& other) noexcept {
        using std::swap;
        swap(root, other.root);
        swap(comp, other.comp);
    }

    int size() const { return subtree_size(root); }
    int unique_size() const { return subtree_distinct_size(root); }
    bool empty() const { return root == nullptr; }

    void clear() {
        recycle_impl(root);
        root = nullptr;
    }

    void insert(T key, int multiplicity = 1) {
        assert(multiplicity > 0);
        bool new_key = false;
        root = insert_impl(root, key, multiplicity, new_key);
    }

    bool erase_one(const T& key) {
        int erased = 0;
        bool removed_key = false;
        root = erase_impl(root, key, false, erased, removed_key);
        return erased != 0;
    }

    bool erase(const T& key) { return erase_one(key); }

    int erase_all(const T& key) {
        int erased = 0;
        bool removed_key = false;
        root = erase_impl(root, key, true, erased, removed_key);
        return erased;
    }

    bool contains(const T& key) const { return count(key) > 0; }
    int count(const T& key) const { return count_impl(root, key); }

    const T* find_by_order(int k) const {
        assert(0 <= k && k < size());
        return kth_impl(root, k);
    }

    T kth(int k) const { return *find_by_order(k); }
    int order_of_key(const T& key) const { return order_of_key_impl(root, key, false); }
    int count_less(const T& key) const { return order_of_key(key); }
    int count_less_equal(const T& key) const { return order_of_key_impl(root, key, true); }
    int count_greater(const T& key) const { return size() - count_less_equal(key); }
    int count_greater_equal(const T& key) const { return size() - count_less(key); }
    const T* lower_bound(const T& key) const { return lower_bound_impl(root, key, false); }
    const T* upper_bound(const T& key) const { return lower_bound_impl(root, key, true); }
    const T* min_ge(const T& key) const { return lower_bound(key); }
    const T* min_gt(const T& key) const { return upper_bound(key); }
    const T* max_le(const T& key) const { return max_less_impl(root, key, false); }
    const T* max_lt(const T& key) const { return max_less_impl(root, key, true); }
    const T* min() const { return empty() ? nullptr : kth_impl(root, 0); }
    const T* max() const { return empty() ? nullptr : kth_impl(root, size() - 1); }

    std::pair<OrderedMultiset, OrderedMultiset> split(const T& key) && {
        auto [l, r] = split_nodes(root, key);
        root = nullptr;
        return {OrderedMultiset(l, comp), OrderedMultiset(r, std::move(comp))};
    }

    OrderedMultiset merge(OrderedMultiset other) && {
        assert(empty() || other.empty() || comp(*max(), *other.min()));
        root = merge_nodes(root, other.root);
        other.root = nullptr;
        return std::move(*this);
    }

    std::vector<T> to_vector() const {
        std::vector<T> result;
        result.reserve(size());
        dump_impl(root, result);
        return result;
    }
};

}  // namespace ds
}  // namespace m1une

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