Given the `root` of a binary tree, return the average value of the nodes on each level in the form of an array. Answers within `10-5` of the actual answer will be accepted.

Example 1:

    Input: root = [3,9,20,null,null,15,7]
    Output: [3.00000,14.50000,11.00000]
    Explanation: The average value of nodes on level 0 is 3, on level 1 is 14.5, and on level 2 is 11.
    Hence return [3, 14.5, 11].

Example 2:

    Input: root = [3,9,20,15,7]
    Output: [3.00000,14.50000,11.00000]

Constraints:

- The number of nodes in the tree is in the range `[1, 104]`.
- `-231 <= Node.val <= 231 - 1`