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root of a binary tree, return the level order traversal of its nodes' values. (i.e., from left to right, level by level).
Input: root = [3,9,20,null,null,15,7] Output: [,[9,20],[15,7]]
Input: root =  Output: []
Input: root =  Output: 
- The number of nodes in the tree is in the range
-1000 <= Node.val <= 1000