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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).
Input is given as the root node of a binary tree.
Return a list of lists of integers representing the zigzag level order traversal of the nodes' values.
The number of nodes in the tree is in the range [0, 2000]. -100 <= Node.val <= 100
Example 1
Input
[3,9,20,null,null,15,7]
Output
[[3],[20,9],[15,7]]
Explanation
Zigzag traversal goes left to right on the first level, right to left on the second, and left to right on the third.