Level Order Traversal

Problem:

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

Examples:

Given binary tree [3,9,20,null,null,15,7],

					
 3
/ \
9  20
  /  \
 15   7

#Return its level order traversal as:
[
  [3],
  [9,20],
  [15,7]
]
				

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