0958. Check Completeness of a Binary Tree

https://leetcode.com/problems/check-completeness-of-a-binary-tree

Description

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 2h nodes inclusive at the last level h.

Example 1:

**Input:** root = [1,2,3,4,5,6]
**Output:** true
**Explanation:** Every level before the last is full (ie. levels with node-values {1} and {2, 3}), and all nodes in the last level ({4, 5, 6}) are as far left as possible.

Example 2:

**Input:** root = [1,2,3,4,5,null,7]
**Output:** false
**Explanation:** The node with value 7 isn't as far left as possible.

Constraints:

• The number of nodes in the tree is in the range [1, 100].

• 1 <= Node.val <= 1000

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