Given a binary tree, return the level order traversal of its nodes' values. (i.e., from left to right, level by level).
Example:
Input: [3, 9, 20, null, null, 15, 7]
3
/ \
9 20
/ \
15 7
Output:
[
[3],
[9, 20],
[15, 7]
]
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