将节点添加到二进制搜索树

Question

The following code should add a node to my BST structure, but fails to do so. New nodes simply dont get attached to the tree, including the root node. Could anyone point out where the problem is?

The constructor for the tree:

template <typename K, typename T>
MyBinaryTree<K, T>::MyBinaryTree() {
    root = nullptr;
}

Function that gets called from the outside:

    template <typename K, typename T>
void MyBinaryTree<K, T>::addValue(K key, T value) {
    Node *node = new Node(key, value);
    addToSubtree(root, node);
}

And the internal private function which has to attach the node:

    template <typename K, typename T>
void MyBinaryTree<K, T>::addToSubtree(MyBinaryTree<K, T>::Node *node,
            MyBinaryTree<K, T>::Node *toAdd) {
    if(node == nullptr) {
        node = toAdd;
    } else {
        if(toAdd->key > node->key) addToSubtree(node->right, toAdd);
        else if(toAdd->key < node->key) addToSubtree(node->left, toAdd);
    }
}

Answer 1

You should use references, like this: MyBinaryTree<K, T>::Node *&root

because of when you modify root in addToSubtree , it modify only local function parameter root, copy of root parameter, not root parameter by itself.

Answer 2

Your node variable is a local variable containing pointer to Node , when you assign a value to it, it only works for that local variable in function scope.

To make your function work, you should make a node variable pointer to pointer (so it gets called like addToSubtree(&node->right, toAdd) or reference to pointer.

Answer 3

Your code suffers from two problems:

1. Although it's supposed to represent Object-Oriented thinkig, the code falls back to C-style of doing things. the root element should not be passed as argument. it exists automaticallty in every member function scope since it's a member variable.

this :

   template <typename K, typename T>
void MyBinaryTree<K, T>::addToSubtree(MyBinaryTree<K, T>::Node *root,
            MyBinaryTree<K, T>::Node *toAdd) {
   //rest...
}

should be this :

   template <typename K, typename T>
void MyBinaryTree<K, T>::addToSubtree( MyBinaryTree<K, T>::Node *toAdd) {
    if(root == nullptr) {
        root = toAdd;
    } //rest...
}

The function already knows what root is since root is a member variable. member functions can change member variables without of them needing to be passed into the function

2. The real problem is the difference between pointer to pointer to pointer . whenever we want a function to change one of the arguments given, that argument should be passed either as pointer or as a reference.

if the argument is already a pointer ,in order to change the pointer itself it should be passed either as pointer to pointer or a referene to pointer

so, let's assume we do want to continue pass root as an argument (we don't), we should pass it either as

void MyBinaryTree<K, T>::addToSubtree(MyBinaryTree<K, T>::Node **root,
            MyBinaryTree<K, T>::Node *toAdd) {
    if(*root == nullptr) {
        *root = toAdd;
        //rest

either as

void MyBinaryTree<K, T>::addToSubtree(MyBinaryTree<K, T>::Node*& root,
            MyBinaryTree<K, T>::Node *toAdd) {
    if(root == nullptr) {
        root = toAdd;
       //the rest...

the second is more C++ style, the first is more C style, but both are valid.