Given 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

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]
]