Binary Tree Level Order Traversal

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).
Example
Given binary tree {3,9,20,#,#,15,7},
    3
   / \
  9  20
     /  \
     15   7
return its level order traversal as:
[
[3],
[9,20],
[15,7]
]

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