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 level h
.
Design an algorithm that runs in less than O(n)
time complexity.
Example 1:
1
/ \
2 3
/ \ /
4 5 6
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.

