Is there any java solution to reduce the time to traverse preorder binary tree?

When I decide to traverse the binary tree after the order for the first time, I only use one stack to store the node and use one List as the result. So the time complexity is obviously N ^ 2. But if you use 2 stacks, the time complexity can be simplified to N.

As far as order traversal goes, I think one stack is enough. But my decision time is still average.

So is there any solution to improve it? Below is my code:

public class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {

        Stack<TreeNode> nodes = new Stack<TreeNode>();
        ArrayList<Integer> result = new ArrayList<Integer>();

        if (root == null)
            return result;
        nodes.push(root);

        while (!nodes.isEmpty()) {
            TreeNode temp = nodes.pop();
            result.add(temp.val);

            if (temp.right != null) {
                nodes.push(temp.right);
            }
            if (temp.left != null) {
                nodes.push(temp.left);
            }
        }

        return result;
    }
}

      

        
        
        
      

    

Thank!