STL 底层揭秘：map/set 如何封装红黑树及迭代器实现

注意，map 的 [] 操作符可以插入新键或访问已有键，这与 string 的 [] 行为不同。此外，库中的红黑树通常包含一个头节点，其 left 指向最小节点，right 指向最大节点，根节点的 parent 也是头节点，这简化了边界处理。

底层红黑树的模拟实现

红黑树的节点结构包含左右子节点指针、父节点指针、数据域和颜色标记。插入时需要维护红黑树的性质，特别是当新节点为红色且父节点也为红色时，需要通过变色或旋转来修复。

enum Colour { RED, BLACK };

template<class T>
struct RBTreeNode {
    RBTreeNode<T>* _left;
    RBTreeNode<T>* _right;
    RBTreeNode<T>* _parent;
    T _data;
    Colour _col;

    RBTreeNode(const T& data) : _left(nullptr), _right(nullptr), _parent(nullptr), _data(data), _col(RED) {}
};

template<class K, class T, class KeyOfT>
struct RBTree {
    typedef RBTreeNode<T> Node;
public:
    typedef __TreeIterator<T, T*, T&> iterator;
    typedef __TreeIterator<T, const T*, const T&> const_iterator;

    iterator begin() {
        Node* leftMin = _root;
        while (leftMin && leftMin->_left)
            leftMin = leftMin->_left;
        return iterator(leftMin);
    }

    iterator end() {
        return iterator(nullptr);
    }

    const_iterator begin() const {
        Node* leftMin = _root;
        while (leftMin && leftMin->_left)
            leftMin = leftMin->_left;
        return const_iterator(leftMin);
    }

    const_iterator end() const {
        return const_iterator(nullptr);
    }

    Node* Find(const K& key) {
        Node* cur = _root;
        KeyOfT kot;
        while (cur) {
            if (kot(cur->_data) < key)
                cur = cur->_right;
            else if (kot(cur->_data) > key)
                cur = cur->_left;
            else
                return cur;
        }
        return nullptr;
    }

    std::pair<iterator, bool> Insert(const T& data) {
        if (_root == nullptr) {
            _root = new Node(data);
            _root->_col = BLACK;
            return std::make_pair(iterator(_root), true);
        }

        Node* parent = nullptr;
        Node* cur = _root;
        KeyOfT kot;
        while (cur) {
            if (kot(cur->_data) < kot(data)) {
                parent = cur;
                cur = cur->_right;
            } else if (kot(cur->_data) > kot(data)) {
                parent = cur;
                cur = cur->_left;
            } else {
                return std::make_pair(iterator(cur), false);
            }
        }

        cur = new Node(data);
        cur->_col = RED;
        if (kot(parent->_data) < kot(data))
            parent->_right = cur;
        else
            parent->_left = cur;
        cur->_parent = parent;

        while (parent && parent->_col == RED) {
            Node* grandfather = parent->_parent;
            if (parent == grandfather->_left) {
                Node* uncle = grandfather->_right;
                // u 存在且为红
                if (uncle && uncle->_col == RED) {
                    parent->_col = uncle->_col = BLACK;
                    grandfather->_col = RED;
                    cur = grandfather;
                    parent = cur->_parent;
                } else {
                    // u 不存在 或 存在且为黑
                    if (cur == parent->_left) {
                        RotateR(grandfather);
                        parent->_col = BLACK;
                        grandfather->_col = RED;
                    } else {
                        RotateL(parent);
                        RotateR(grandfather);
                        cur->_col = BLACK;
                        grandfather->_col = RED;
                    }
                    break;
                }
            } else {
                Node* uncle = grandfather->_left;
                if (uncle && uncle->_col == RED) {
                    parent->_col = uncle->_col = BLACK;
                    grandfather->_col = RED;
                    cur = grandfather;
                    parent = cur->_parent;
                } else {
                    if (cur == parent->_right) {
                        RotateL(grandfather);
                        grandfather->_col = RED;
                        parent->_col = BLACK;
                    } else {
                        RotateR(parent);
                        RotateL(grandfather);
                        cur->_col = BLACK;
                        grandfather->_col = RED;
                    }
                    break;
                }
            }
        }
        _root->_col = BLACK;
        return std::make_pair(iterator(cur), true);
    }

private:
    Node* _root = nullptr;
    // 旋转逻辑参考 AVL 树实现
    void RotateL(Node* parent) {}
    void RotateR(Node* parent) {}
};

迭代器的模拟实现

迭代器的核心在于 ++ 和 -- 的操作。begin 指向最左节点，end 指向 nullptr（或头节点）。++ 操作时，如果右子树不为空，则找右子树的最左节点；否则向上回溯，直到当前节点是其父节点的左孩子。-- 操作逻辑相反，优先找左子树的最右节点，否则向上回溯直到是当前节点的右孩子。

template<class T, class Ptr, class Ref>
struct __TreeIterator {
    typedef RBTreeNode<T> Node;
    typedef __TreeIterator<T, Ptr, Ref> Self;
    typedef __TreeIterator<T, T*, T&> Iterator;

    __TreeIterator(const Iterator& it) : _node(it._node) {}
    Node* _node;

    __TreeIterator(Node* node) : _node(node) {}

    Ref operator*() {
        return _node->_data;
    }

    Ptr operator->() {
        return &_node->_data;
    }

    bool operator!=(const Self& s) const {
        return _node != s._node;
    }

    bool operator==(const Self& s) const {
        return _node == s._node;
    }

    Self& operator++() {
        if (_node->_right) {
            Node* subLeft = _node->_right;
            while (subLeft->_left)
                subLeft = subLeft->_left;
            _node = subLeft;
        } else {
            Node* cur = _node;
            Node* parent = cur->_parent;
            while (parent && cur == parent->_right) {
                cur = cur->_parent;
                parent = parent->_parent;
            }
            _node = parent;
        }
        return *this;
    }

    Self& operator--() {
        if (_node->_left) {
            Node* subRight = _node->_left;
            while (subRight->_right)
                subRight = subRight->_right;
            _node = subRight;
        } else {
            Node* cur = _node;
            Node* parent = cur->_parent;
            while (parent && cur == parent->_left) {
                cur = cur->_parent;
                parent = parent->_parent;
            }
            _node = parent;
        }
        return *this;
    }
};

普通迭代器可以隐式转换为 const 迭代器，这保证了接口的一致性。理解这些底层细节，有助于在面试中清晰阐述 STL 原理，也能在实际开发中更好地利用容器特性。