LeetCode-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
  
    preorderTraversal(TreeNode root) {
        List
   
     result=new ArrayList
    
     (); Stack
     
       stack=new Stack
      
       (); TreeNode node=root; do { while(node!=null) { result.add(node.val); if(node.right!=null) { stack.push(node.right); } node=node.left; } if(!stack.isEmpty()) { node=stack.pop(); } }while(!stack.isEmpty() || node!=null); return result; } }