Count Complete Tree Nodes

Easy

Given the root of a binary tree, return the total number of nodes.

Example 1

Input:  root = [1,2,3,4,5,6]
Output: 6

Example 2

Input:  root = []
Output: 0

Constraints

• 0 <= number of nodes <= 5*10^4
• -10^4 <= Node.val <= 10^4