如何正确添加二叉搜索树中的节点？

Question

This is my code

public boolean insertWord(String key, String meaning) {

    if((root == null) ){
        root = new TreeNode();

        root.key = key;
        root.meaning = meaning;
    }


    else{

        TreeNode subroot = root;

        if(subroot.key.compareTo(key) == 0){
            return false;
        }
        else if(key.compareTo(subroot.key) < 0){

            if(subroot.left != null){
                subroot = root.left;
                return insertWord(key, meaning);
            }
            else{
                subroot.left = new TreeNode();
                subroot.left.key = key;
                subroot.left.meaning = meaning;
            }

        }
        else{
            if(subroot.right != null){
                subroot = root.right;
                return insertWord(key, meaning);
            }
            else{
                subroot.right = new TreeNode();
                subroot.right.key = key;
                subroot.right.meaning = meaning;
            }

        }
    }

    return true;
}

Doing this gives me stackoverflow error. Can someone please help me understand why I keep getting that error. I know its because of an infinite loop but I dont know why its happening. Can someone tell me where it is happening and how to fix it? Thanks

Answer 1

In the below code if subroot is set as root.left then shouldn't you use the key of subroot further? Where are you passing that information?

if(subroot.left != null){
       subroot = root.left;
       return insertWord(key, meaning);
 }

Now I am presenting my version which I have implemented:

protected Node<T> insertValue(T value) {
    Node<T> newNode = getNewNode(value);

    // If root is null, assign
    if (root == null) {
        root = newNode;
        size++;
        return newNode;
    }

    Node<T> currentNode = root;
    while (currentNode != null) {
        if (newNode.getData().compareTo(currentNode.getData()) <= 0) {  // Less than or equal to goes left
            if(currentNode.getLeft() == null) {
                insertNodeToLeft(currentNode, newNode);
                break;
            }
            currentNode = currentNode.getLeft();
        } else {                                        // Greater than goes right
            if (currentNode.getRight() == null) {
                insertNodeToRight(currentNode, newNode);
                break;
            }
            currentNode = currentNode.getRight();
        }
    }

    return newNode;
}

Hope it will help you.

Answer 2

As Aaron has pointed out you need to update the new key to which you will compare next. In your code if left node is null you inserted the node but if it is not null then you need to compare your key with the key of this new node. Where is this code?

else if(key.compareTo(subroot.key) < 0){

            if(subroot.left != null){
                subroot = root.left;
            // WHERE ARE YOU USING KEY OF THIS NEW NODE subroot FOR COMPARISON? 
            return insertWord(key, meaning);
        }
        else{
            subroot.left = new TreeNode();
            subroot.left.key = key;
            subroot.left.meaning = meaning;
        }

    }

Edit: The implementation for methods to insert left and right should be something similar:

private void insertNodeToLeft(Node<T> parent, Node<T> child) {
    // New left node
    parent.setLeft(child);
    child.setParent(parent);
    size++;
}

private void insertNodeToRight(Node<T> parent, Node<T> child) {
    // New right node
    parent.setRight(child);
    child.setParent(parent);
    size++;
}