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Given a binary tree (level-order array), return the level order traversal as each level on a separate line.
Output each level space-separated.
3,9,20,null,null,15,7
3 9 20 15 7
3 is the root, the next two elements 9 and 20 are its children, and the last two elements 15 and 7 are the children of 20.3, which is the first level and only contains the value 3.9 and 20, which are output as 9 20.20, which are 15 and 7, output as 15 7.