Given the root of a binary tree, return the zigzag level order traversal of its nodes' values. The zigzag traversal is done from left to right, then right to left for the next level and alternate between.

Example

Input: Given binary tree [5,8,2,null,null,6,4]
    5
   / \
  8   2
     / \
    6   4

Output: Return its zigzag level order traversal as:
[
  [5],
  [2,8],
  [6,4]
]