Binary Tree Level Order Traversal
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Given the root of a binary tree, return the level order traversal of its nodes' values (i.e., from left to right, level by level).
Example:
Input: root = [3,9,20,null,null,15,7]
Output: [[3],[9,20],[15,7]]
Examples
Input → {"val":3,"left":{"val":9},"right":{"val":20,"left":{"val":15},"right":{"val":7}}}
Output → [[3],[9,20],[15,7]]
Input → {"val":1}
Output → [[1]]
Input → No value
Output → []