Given the `root` of a binary tree, determine if it is a complete binary tree.

In a complete binary tree, every level, except possibly the last, is completely filled, and all nodes in the last level are as far left as possible. It can have between `1` and `2h` nodes inclusive at the last level `h`.

Example 1:

    Input: root = [1,2,3,4,5,6]
    Output: true
    Explanation: Every level before the last is full (ie. levels with node-values {1} and {2, 3}), and all nodes in the last level ({4, 5, 6}) are as far left as possible.

Example 2:

    Input: root = [1,2,3,4,5,null,7]
    Output: false
    Explanation: The node with value 7 isn't as far left as possible.

Constraints:

- The number of nodes in the tree is in the range `[1, 100]`.
- `1 <= Node.val <= 1000`