Binary Tree Level Order Traversal
Return the level order traversal as a list of lists Example: [3,9,20,null,null,15,7] => [[3],[9,20],[15,7]]
Binary Tree Level Order Traversal is a traversal problem: the hard part is modeling the state you visit. Interviewers love it because it reveals your traversal modeling and your edge-case discipline. Once your traversal structure is solid, you can reuse it across dozens of problems.
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