如何将 BST 自上而下、从左到右插入到 ArrayList 中？

Question

I know how to insert a BST into an ArrayList in order [25, 50, 100, 125, 150, 180]. What I would like to do is insert the tree into an ArrayList in the way it is represented in a tree. For example, [100, 50, 150, 25, null, 125, 180] would be a tree that looks like this:

                                     100
                                   /      \
                                 50       150
                               /   \     /   \
                             25    null 125  180

I would like to be able to put the tree into an ArrayList in this way, including the nulls. How can I do this?

Here is the code I have now. It will print the tree values but not the null values:

private int height(Node<T> current){
    if(current == null){
        return 0;
    }
    else{
        //
        int leftHeight = height(current.left);
        int rightHeight = height(current.right);

        //
        if (leftHeight > rightHeight) {
            return (leftHeight + 1);
        }
        else{
            return (rightHeight + 1);
        }
    }
}

private void printAllLevels(Node<T> current, int level){
    if (current == null){
        return;
    }
    if(level == 1){
        System.out.print(current.data + " ");
    }
    else if (level > 1) {
        printAllLevels(current.left, level - 1);
        printAllLevels(current.right, level - 1);
    }
}

@Override
public String toString() {
    toString(this.root);
    return null;
}


private void toString(Node<T> current){
    int h = height(this.root);
    for(int i = 1; i <= h; i++){
        printAllLevels(this.root, i);
    }
}

Answer 1

private int height(Node<T> current){
    if(current == null){
        return 0;
    }
    else{
        //
        int leftHeight = height(current.left);
        int rightHeight = height(current.right);

        //
        if (leftHeight > rightHeight) {
            return (leftHeight + 1);
        }
        else{
            return (rightHeight + 1);
        }
    }
}

private void printAllLevels(Node<T> current, int level){
    if (current == null){
        System.out.print("null ");
        return;
    }
    if(level == 1){
        System.out.print(current.data + " ");
    }
    else if (level > 1) {
        printAllLevels(current.left, level - 1);
        printAllLevels(current.right, level - 1);
    }
}

@Override
public String toString() {
    toString(this.root);
    return null;
}


private void toString(Node<T> current){
    int h = height(this.root);
    for(int i = 1; i <= h; i++){
        printAllLevels(this.root, i);
    }
}