0222. Count Complete Tree Nodes

https://leetcode.com/problems/count-complete-tree-nodes

Description

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.

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