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

9 3 15 20 7