Two-Three Tree

public void insert(K key) {
        if(this.root == null){
            this.root = new TreeNode<>(key);
            return;
        }

        TreeNode<K> newRoot = this.insert(this.root, key);

        if(newRoot != null){
            this.root = newRoot;
        }
    }

    private TreeNode<K> insert(TreeNode<K> node, K key) {
        if(node.isLeaf()){
            if(node.isTwoNode()){
                if(node.leftKey.compareTo(key) < 0){
                    node.rightKey = key;
                } else{
                    node.rightKey = node.leftKey;
                    node.leftKey = key;
                }

                return null;
            }
            K left = node.leftKey;
            K middle = key;
            K right = node.rightKey;

            if(middle.compareTo(left) < 0){
                left = key;
                middle = node.leftKey;
            } else if(middle.compareTo(right) > 0){
                right = key;
                middle = node.rightKey;
            }

            return new TreeNode<>(middle, left, right);
        }

        TreeNode<K> toFix = null;
        if(node.isTwoNode() && node.leftKey.compareTo(key) > 0){
            toFix = this.insert(node.leftChild, key);
        } else if(node.isTwoNode() && node.leftKey.compareTo(key) < 0){
            toFix = this.insert(node.rightChild, key);
        } else if(node.isThreeNode() && node.leftKey.compareTo(key) > 0){
            toFix = this.insert(node.leftChild, key);
        } else if(node.isThreeNode() && node.rightKey.compareTo(key) < 0){
            toFix = this.insert(node.rightChild, key);
        } else {
            toFix = this.insert(node.middleChild, key);
        }

        if(toFix == null) return null;

        if(node.isTwoNode()){
            if(toFix.leftKey.compareTo(node.leftKey) < 0){
                node.rightKey = node.leftKey;
                node.leftKey = toFix.leftKey;

                node.leftChild = toFix.leftChild;
                node.middleChild = toFix.rightChild;
            } else{
                node.rightKey = toFix.leftKey;

                node.middleChild = toFix.leftChild;
                node.rightChild = toFix.rightChild;
            }

            return null;
        }

        K promoteValue = null;
        TreeNode<K> left = null;
        TreeNode<K> right = null;

        if(toFix.leftKey.compareTo(node.leftKey) < 0){
            promoteValue = node.leftKey;
            left = toFix;
            right = new TreeNode<>(node.rightKey, node.middleChild, node.rightChild);
        } else if(toFix.leftKey.compareTo(node.rightKey) > 0){
            promoteValue = node.rightKey;
            left = new TreeNode<>(node.leftKey, node.leftChild, node.middleChild);
            right = toFix;
        } else{
            promoteValue = toFix.leftKey;
            left = new TreeNode<>(node.leftKey, node.leftChild, toFix.leftChild);
            right = new TreeNode<>(node.rightKey, toFix.rightChild, node.rightChild);
        }

        return new TreeNode<>(promoteValue, left, right);
    }