LeetCode 144: Binary Tree Preorder Traversal

Given a binary tree, return the preorder traversal of its nodes' values.
For example:
Given binary tree {1,#,2,3},

   1
    \
     2
    /
   3

return [1,2,3].
Note: Recursive solution is trivial, could you do it iteratively?

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> pre = new LinkedList<>();
        Stack<TreeNode> stack = new Stack<>();
        
        TreeNode node = root;

        while (!stack.isEmpty() || node != null)
        {
            if (node != null)
            {
                // Center
                pre.add(node.val);
                
                if (node.right != null)
                    stack.push(node.right);
                
                // Left
                node = node.left;
            }
            else
                // Right
                node = stack.pop();
        }
        
        return pre;
    }
}