Why is my preorder traversal returning an empty list?

Question

I am trying to understand why code is returning an empty list. I am not looking for a solution to the problem. The solution already exists in What is wrong with my Preorder traversal?

I want to understand, what is incorrect about my current understanding of recursion and the call stack upon using a global variable.

class Solution {
    ArrayList<Integer> list;
    public List<Integer> preorderTraversal(TreeNode root) {
        list = new ArrayList<Integer>();
        preOrder(root);
        return list;
    }
    public void preOrder(TreeNode node){
        System.out.println(list.toString());
        if(node == null)
            return;
        
        list.add(node.val);
        
        //System.out.println(list.toString());
        
        preorderTraversal(node.left);
        preorderTraversal(node.right);
    }
}

For the following tree : [1,null,2,3]

I notice that in the first print call I get 7 empty lists

[]
[]
[]
[]
[]
[]
[]

In my second print call, I get the following

[1]
[2]
[3]

Why isn't my global variable list not "saving" or "appending" to the next call stack? It seems like, there are multiple list objects being utilized.

Answer 1

Because you use preorderTraversal with left and right nodes so each time it will override list value with a new empty list, you should use preOrder with left and right node like this

public void preOrder(TreeNode node){
    if(node == null)
        return;
        
    list.add(node.val);
        
    preOrder(node.left);
    preOrder(node.right);
}