Binary Tree Inorder Traversal

    //iterative
    public ArrayList<Integer> inorderTraversal(TreeNode root) {
        // Start typing your Java solution below
        // DO NOT write main() function
        ArrayList<Integer> result = new ArrayList<Integer>();
        if(root == null) return result;
        Stack<TreeNode> stack = new Stack<TreeNode>();
        stack.push(root);
        while(!stack.empty()) {
            root = stack.peek();
            if(root.left != null) {
                TreeNode tmp = root.left;
                root.left = null;
                stack.push(tmp);
            }else {
                root = stack.pop();
                result.add(root.val);
                if(root.right != null) stack.push(root.right);
            }
        }
        return result;
    }



    //recursive
    public ArrayList<Integer> inorderTraversal(TreeNode root) {
        // Start typing your Java solution below
        // DO NOT write main() function
        ArrayList<Integer> result = new ArrayList<Integer>();
        help(root, result);
        return result;
    }
    
    public void help(TreeNode root, ArrayList<Integer> result) {
        if(root == null) return;
        if(root.left != null) help(root.left, result);
        result.add(root.val);
        if(root.right != null) help(root.right, result);
    }