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).
Example:
Input: root = [3, 9, 20, null, null, 15, 7]
3
/ \
9 20
/ \
15 7
Output:
[
[3],
[20,9],
[15,7]
]
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Solution #2 · O(n) Iterative
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