public class Solution
extends Object
222 - Count Complete Tree Nodes\.
Medium
Given the `root` of a **complete** binary tree, return the number of the nodes in the tree.
According to **[Wikipedia](http://en.wikipedia.org/wiki/Binary_tree#Types_of_binary_trees)** , 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**.