Binary Tree Level Order Traversal

Medium

Given the root of a binary tree, return its level-order traversal — a list of levels, each a list of node values from left to right. On this judge levels are shown separated by |.

Example 1

Input:  root = [3,9,20,null,null,15,7]
Output: [[3],[9,20],[15,7]]

Example 2

Input:  root = []
Output: []

Constraints

• 0 <= number of nodes <= 2000
• -1000 <= Node.val <= 1000