Given the `root` of a complete binary tree, return the number of the nodes in the tree.

According to Wikipedia, every level, except possibly the last, is completely filled in a complete binary tree, 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`.

Design an algorithm that runs in less than `O(n)` time complexity.

Example 1:

    Input: root = [1,2,3,4,5,6]
    Output: 6

Example 2:

    Input: root = []
    Output: 0

Example 3:

    Input: root = [1]
    Output: 1

Constraints:

- The number of nodes in the tree is in the range `[0, 5 * 104]`.
- `0 <= Node.val <= 5 * 104`
- The tree is guaranteed to be complete.