https://leetcode.com/problems/maximum-width-of-binary-tree/

Given a binary tree, write a function to get the maximum width of the given tree. The width of a tree is the maximum width among all levels. The binary tree has the same structure as a **full binary tree**, but some nodes are null.

The width of one level is defined as the length between the end-nodes (the leftmost and right most non-null nodes in the level, where the `null`

nodes between the end-nodes are also counted into the length calculation.

**Example 1:**

Input:1 / \ 3 2 / \ \ 5 3 9Output:4Explanation:The maximum width existing in the third level with the length 4 (5,3,null,9).