Leetcode 103
https://leetcode.com/problems/binary-tree-zigzag-level-order-traversal/description/
Given the root of a binary tree, return the zigzag level order traversal of its nodes' values. (i.e., from left to right, then right to left for the next level and alternate between).
Example 1:
Input: root = [3,9,20,null,null,15,7] Output: [[3],[20,9],[15,7]] Example 2:
Input: root = [1] Output: [[1]] Example 3:
Input: root = [] Output: []
Constraints:
The number of nodes in the tree is in the range [0, 2000]. -100 <= Node.val <= 100