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).
[0, 2000]-100 <= Node.val <= 100root = [3,9,20,null,null,15,7][[3],[20,9],[15,7]]root = [1][[1]]root = [][]