841. Count Total Nodes in Binary Tree

Given the root of a binary tree, return the total number of nodes in it. The tree is given as a level-order array with null for missing children.

Input: A level-order array of the tree.

Output: Integer — the node count.

Examples

Example 1

Input: [1,2,3,4,5,null,6]
Output: 6
Explanation: Six nodes total.

Example 2

Input: [1]
Output: 1
Explanation: Single node.

Example 3

Input: []
Output: 0
Explanation: Empty tree.