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