222. Count Complete Tree Nodes

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**h nodes inclusive at the last leve h.

Design an algorithm that runs in less than 0(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.

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