Binary Tree Level Order Traversal Medium

Given the root of a binary tree (TreeNode with val, left, right; null for empty), return the node values grouped level by level, from left to right, as an array of arrays.

Examples

• [3,9,20,null,null,15,7] -> [[3],[9,20],[15,7]]
• [] -> []

Define a function named levelOrder in JavaScript. TreeNode (val, left, right) is available.