Binary Tree Preorder Traversal - LeetCode

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 {    private ArrayList<Integer> result = new ArrayList<Integer>();    public ArrayList<Integer> preorder(TreeNode root){        if(root == null)        return result;        else {            result.add(root.val);            preorder(root.left);            preorder(root.right);            return result;        }    }    public ArrayList<Integer> preorderTraversal(TreeNode root) {        preorder(root);        return result;    }}