Trouble balancing the root node in an AVL tree

Question

Hello guys i seem to be having trouble rebalancing the AVL tree. It balances everything else except the root node. I don't know why it doesn't balance the root node. When i pass values such as 50, 40, 30, 20, and 10. I get 50,30, 20, 10 and 40. 50 should not be the root, the correct root value should be 40. Below is my code:

public void insert(T data){

    AVLNode<T> newNode = new AVLNode<T>(data);
    if(isEmpty()){
        root = newNode;
    }
    else{
        insert(root, newNode);
    }

}

private void insert(AVLNode<T> root, AVLNode<T> newNode){

 if(newNode.getData().compareTo(root.getData())<0){
        if(root.getLeftChild()!=null){
        AVLNode<T> leftNodes = root.getLeftChild();
        insert(leftNodes, newNode);
        root.setLeftChild(rebalance(leftNodes));
        }
        else{
            root.setLeftChild(newNode);
        }
    }
    else if(newNode.getData().compareTo(root.getData())>0){
        if(root.getRightChild()!=null){
        AVLNode<T> rightNodes = root.getRightChild();
        insert(rightNodes, newNode);
        root.setRightChild(rebalance(rightNodes));
    }
        else{
            root.setRightChild(newNode);
        }
    }
    else{
          root.setData(newNode.getData());
    }
    updateHeight(root); 
}

//re-balances the tree.
private AVLNode<T> rebalance(AVLNode<T> root){
    int difference = balance(root);
    if (difference > 1){
        if(balance(root.getLeftChild())>0){
            root = rotateRight(root);
        }
        else{
            root = rotateLeftRight(root);
        }
    }
    else if(difference < -1){
        if(balance(root.getRightChild())<0){
            root = rotateLeft(root);
        }
        else{
            root = rotateRightLeft(root);
        }
    }
    return root;
}

//updates the height of the tree.
public void updateHeight(AVLNode<T> root){
    if((root.getLeftChild()==null) && (root.getRightChild()!=null)){
        root.setHeight(root.getRightChild().getHeight()+1);
    }
    else if((root.getLeftChild() !=null)&&(root.getRightChild()==null)){
        root.setHeight(root.getLeftChild().getHeight()+1);
    }
    else
        root.setHeight(Math.max(getHeight(root.getLeftChild()), getHeight(root.getRightChild()))+1);
}

private int balance(AVLNode<T> root) {
    return getHeight(root.getLeftChild())-getHeight(root.getRightChild());
}

//single left left rotation of the tree
private AVLNode<T> rotateLeft(AVLNode<T> root){
    AVLNode<T> NodeA = root.getRightChild();
    root.setRightChild(NodeA.getLeftChild());
    NodeA.setLeftChild(root);
    root.setHeight(Math.max(getHeight(root.getLeftChild()), getHeight(root.getRightChild()))+1); //updates the height
    NodeA.setHeight(Math.max(getHeight(NodeA.getLeftChild()), getHeight(NodeA.getRightChild()))+1);
    return NodeA;
}
//single right right rotation of the tree
private AVLNode<T> rotateRight(AVLNode<T> root){
    AVLNode<T> NodeA = root.getLeftChild();
    root.setLeftChild(NodeA.getRightChild());
    NodeA.setRightChild(root);
    root.setHeight(Math.max(getHeight(root.getLeftChild()), getHeight(root.getRightChild()))+1); //updates the height of the AVL tree
    NodeA.setHeight(Math.max(getHeight(NodeA.getLeftChild()), getHeight(NodeA.getRightChild()))+1);
    return NodeA;
}
//a double rotation. Left right rotation
private AVLNode<T> rotateLeftRight(AVLNode<T> root){
    AVLNode<T> nodeA = root.getLeftChild();
    root.setLeftChild(rotateLeft(nodeA));
    return rotateRight(root);
}
//a right left rotation
private AVLNode<T> rotateRightLeft(AVLNode<T> root){
    AVLNode<T> nodeA = root.getRightChild();
    root.setRightChild(rotateRight(nodeA));
    return rotateLeft(root);
}

Answer 1

In your insert method, you should consider adding

rebalance(root);

Just before the call

updateHeight(root);

at the end of the method. Then make sure to remove calls to rebalance before this. So, for example, change

root.setLeftChild(rebalance(leftNodes));

to

root.setLeftChild(leftNodes);

Assuming the rest of your code works fine, this should be the fix. Then each of your nodes that is ever considered a local root will have the balancing method called on them.. including the absolute root of the tree.