C++ 实现二叉搜索树(BST)

class BST
{
public:
    struct Node
    {
        int key;//节点的key
        int value;//节点的value
        Node* left;
        Node *right;
        int N;//节点的叶子节点数目
        Node(int _key, int _value, int _N)
        {
            key = _key;
            value = _value;
            N = _N;
        }
    };
    BST();
    ~BST();
    void put(int key, int value);
    int get(int key);
    int deleteKey(int key);
private:
    Node* _deleteKey(int key, Node *x);
    Node* _deleteMin(Node *x);
    int size(Node *x);
    int _get(int key, Node* x);
    Node * _put(int key, int value,Node *x);
    Node * min(Node *x);
    Node* root;
};
inline int BST::size(Node * x)
{
    if (x == nullptr)return 0;
    return x->N;
}
int BST::_get(int key, Node * x)
{
    if (x == nullptr)return 0;
    if (x->key < key)_get(key, x->right);
    else if (x->key > key)_get(key, x->left);
    else {
        return x->value;
    }
    return 0;
}
BST::Node* BST::_put(int key, int value, Node * x)
{
    if (x == nullptr) {
        Node *tmp = new Node(key, value, 1);
        return tmp;
    }
    if (x->key > key) {
        x->left=_put(key, value, x->left);
    }
    else if (x->key < key) {
        x->right=_put(key, value, x->right);
    }
    else x->key = key;  
    x->N = size(x->left) + size(x->right) + 1;
    return x;
}
BST::Node* BST::min(Node * x)
{
    if (x->left == nullptr)return x;
    return min(x->left);
}
BST::BST()
{
}

BST::~BST()
{
}

void BST::put(int key, int value)
{
    root=_put(key, value, root);
}

int BST::get(int key)
{
    return _get(key, root);
}
BST::Node* BST::_deleteKey(int key, Node * x)
{
    if (x->key > key)x->left = _deleteKey(key, x->left);
    else if (x->key < key)x->right = _deleteKey(key, x->right);
    else {
        if (x->left == nullptr)return x->right;
        else if (x->right == nullptr)return x->left;
        else {
            Node *tmp = x;
            x = min(tmp->right);
            x->left = tmp->left;
            x->right = _deleteMin(tmp->right);
        }
    }
    x->N = size(x->left) + size(x->right) + 1;
    return x;
}
BST::Node* BST::_deleteMin(Node * x)
{
    if (x->left == nullptr)return x->right;
    x->left = _deleteMin(x->left);
    x->N = size(x->left) + size(x->right) + 1;
    return x;
}
int BST::deleteKey(int key)
{
    return _get(key, root);
}