How to return an Iterator for Inorder traversal in Binary Tree?

Question

I am trying to store my results of Inorder traversal in an LinkedList and retrieve by iterator, but getting null pointer exception while printing my results. I get correct output when I try to do it by recursion and print value in my function. When I recursively try to call inorderItr(root.left), it takes root as null. I think, my return statement is not correct, Not sure, below is my code and comments where my code is breaking. Any help and concepts are appreciated. I have seen this, but doesnt help, as I am trying to return an Iterator. Again, I am new to Java and Iterator concept. TIA.

Edit: I have found the solution, please see the answer below

  class TreeNode {

            int data;
            TreeNode left;
            TreeNode right;

            public TreeNode(int d) {
                data = d;
            }

        }

        public class TreeTraversal {
             TreeNode root;

            public TreeTraversal() {
                root = null;
            }

       static List<TreeNode> l = new LinkedList<TreeNode>();
            public static Iterator<TreeNode> inorderItr(TreeNode root) {

                List<TreeNode> l = new LinkedList<TreeNode>();

      //I think I am missing something here
                if (root == null)
                    return

      //This is where my root is null
                inorderItr(root.left);
                l.add(root);
                inorderItr(root.right);

                Iterator<TreeNode> itr = l.iterator();

                return itr;

            }

    //This code works fine
            public static void inorderWorksFine(TreeNode root) {

                if (root == null)
                    return;

                inorder(root.left);
                System.out.print(root.data + " ");
                inorder(root.right);
            }



            public static void main(String args[]) {

                TreeTraversal t = new TreeTraversal();
                t.root = new TreeNode(10);
                t.root.left = new TreeNode(5);
                t.root.left.left = new TreeNode(1);
                t.root.left.right = new TreeNode(7);
                t.root.right = new TreeNode(40);
                t.root.right.right = new TreeNode(50);

                // inorderWorksFine(t.root);
                Iterator<TreeNode> itr = inorderItr(t.root);

                while (itr.hasNext()) {
                    System.out.println(itr.next().data + " ");
                }

            }

        }

Answer 1

I have created a helper method for inorder traversal and Global LinkedList and added all my Inorder elements to that list in a separate recursive helper method. That way we can return an Iterator

static List<TreeNode> l = new LinkedList<TreeNode>();

    public static Iterator<TreeNode> inorderItr(TreeNode root) {
    recursionInorder(root);
    Iterator<TreeNode> itr = l.iterator();

     return itr;

    }

    public static void recursionInorder(TreeNode node){
        if(node==null)
              return;

        recursionInorder(node.left);
        l.add(node);
        recursionInorder(node.right);
    }