LeetCode Binary Tree Inorder Traversal

Given a binary tree, return the inorder traversal of its nodes' values.

For example:
Given binary tree {1,#,2,3},

return [1,3,2].

Note: Recursive solution is trivial, could you do it iteratively?

 1 /**
 2  * Definition for binary tree
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 public class Solution {
11     List<Integer> list=new ArrayList<>();
12     public List<Integer> inorderTraversal(TreeNode root) {
13             if (root==null) {
14                 return list;
15             }
16             if (root.left != null) {
17                 inorderTraversal(root.left);
18             }
19             list.add(root.val);
20             if (root.right != null) {
21                 inorderTraversal(root.right);
22             }
23             return list;
24     }
25 }