Check Completeness of a Binary Tree

You are given the root of a binary tree, determine if it is a complete binary tree.

In a complete binary tree, every level, except possibly the last, is completely filled, 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`.

Example 1:

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

Output: true

Example 2:

Input: root = [1,2,3,4,5,null,7]

Output: false

Example 3:

Input: root = [1,2,3,4]

Output: true

Constraints:

• 1 <= The number of nodes in the tree <= 100.
• 1 <= Node.val <= 1000

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