### 103. Binary Tree Zigzag Level Order Traversal

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,null,null,15,7],

    3
/ \
9  20
/  \
15   7


return its zigzag level order traversal as:

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


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