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 `2`

nodes inclusive at the last level ^{h}`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 * 10`

.^{4}] `0 <= Node.val <= 5 * 10`

^{4}- The tree is guaranteed to be
**complete**.