LeetCode Medium: Binary Tree Inorder Traversal

Here are 2 of my accepted solution approaches (both O(n) where n is the number of nodes in the input tree) for solving the Problem #94 Binary Tree Inorder Traversal on Leetcode:

Approach 1: Simple Recursive

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> values = new ArrayList<>();
        preorderTraversalRecur(root, values);
        return values;
    }
    
    void preorderTraversalRecur(TreeNode root, List<Integer> values) {
        if (root == null)
            return;
        
        values.add(root.val);
        preorderTraversalRecur(root.left, values);
        preorderTraversalRecur(root.right, values);
    }
}

Approach 2: Iterative Stack based

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> values = new ArrayList<>();
        inorderTraversal(root, values);
        return values;
    }
    
    void inorderTraversal(TreeNode root, List<Integer> values) {
        if (root == null)
            return;
        
        inorderTraversal(root.left, values);
        values.add(root.val);
        inorderTraversal(root.right, values);
    }
}

Know of any more alternative? ..Feel free to share your code or suggestions in comments below!