public class Solution
extends Object
102 - Binary Tree Level Order Traversal\.
Medium
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`