Binary Tree Inorder Traversal

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

Example:

Input: [1,null,2,3]
   1
    \
     2
    /
   3

Output: [1,3,2]

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

//Recursive solution
class Solution {
    public void helper(TreeNode root, List<Integer> ans) {
        if (root == null)
            return;
        helper(root.left, ans);
        ans.add(root.val);
        helper(root.right, ans);
    }

    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> ans = new ArrayList<>();
        helper(root, ans);
        return ans;
    }
}
//Iterative solution
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        Deque<TreeNode> stack = new LinkedList<>();
        TreeNode p = root;
        while (!stack.isEmpty() || p != null) {
            if (p != null) {
                stack.push(p);
                p = p.left;
            } else {
                TreeNode node = stack.pop();
                result.add(node.val); // Add after all left children
                p = node.right;
            }
        }
        return result;
    }
}