Question:

Given a binary tree, return the zigzag level order traversal of its
nodes' values. (ie, from left to right, then right to left for the next
level and alternate between).

For example:
Given binary tree {3,9,20,#,#,15,7},

    3
   / \
  9  20
    /  \
   15   7

return its zigzag level order traversal as:

[
  [3],
  [20,9],
  [15,7]
]