public class Solution
extends Object
958 - Check Completeness of a Binary Tree\.
Medium
Given the `root` of a binary tree, determine if it is a _complete binary tree_.
In a **[complete binary tree](http://en.wikipedia.org/wiki/Binary_tree#Types_of_binary_trees)** , 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`