Create an  AVL Tree for the given values 21, 26, 30, 9, 4, 14, 28, 18,15,10, 2, 3, 7.  What is the root node element

To create an AVL Tree for the given values 21, 26, 30, 9, 4, 14, 28, 18, 15, 10, 2, 3, and 7, we will follow these steps:

1. Start by inserting the first value, 21, as the root node of the AVL Tree.
2. Insert the next value, 26, as the right child of the root node since it is greater than 21.
3. Insert the next value, 30, as the right child of the node with the value 26 since it is greater than 26.
4. Insert the next value, 9, as the left child of the root node since it is smaller than 21.
5. Insert the next value, 4, as the left child of the node with the value 9 since it is smaller than 9.
6. Insert the next value, 14, as the right child of the node with the value 9 since it is greater than 9.
7. Insert the next value, 28, as the left child of the node with the value 30 since it is smaller than 30.
8. Insert the next value, 18, as the left child of the node with the value 14 since it is smaller than 14.
9. Insert the next value, 15, as the right child of the node with the value 18 since it is greater than 18.
10. Insert the next value, 10, as the left child of the node with the value 14 since it is smaller than 14.
11. Insert the next value, 2, as the left child of the node with the value 4 since it is smaller than 4.
12. Insert the next value, 3, as the right child of the node with the value 2 since it is greater than 2.
13. Insert the last value, 7, as the right child of the node with the value 4 since it is greater than 4.

After following these steps, the AVL Tree will look like this:

        21
       /  \
      9    26
     / \     \
    4   14    30
   / \  /     /
  2   7 10   28
       \
        18
       /
      15

Therefore, the root node element of the AVL Tree is 21.