Binary Tree Level Order Traversal

# Binary Tree Level Order Traversal 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 1 ``` Input: root = [3,9,20,null,null,15,7] Output: [[3],[9,20],[15,7]] ``` ## Example 2 ``` Input: root = [1] Output: [[1]] ``` ## Example 3 ``` Input: root = [] Output: [] ``` ## Constraints - The number of nodes in the tree is in the range `[0, 2000]`. - `-1000 <= Node.val <= 1000`