LeetCoder - 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 ArrayList<Integer> preorderTraversal(TreeNode root) {

        ArrayList<Integer> ret = new ArrayList<Integer>();

        if(root==null) {

            return ret;

        }

        Stack<TreeNode> lst = new Stack<TreeNode>();

        lst.push(root);

        while(!lst.isEmpty()) {

            TreeNode node = lst.pop();

            ret.add(node.val);

            if(node.right!=null) lst.push(node.right);

            if(node.left!=null) lst.push(node.left);

        }

        return ret;

    }

}