107\. Binary Tree Level Order Traversal II

Medium

Given the `root` of a binary tree, return _the bottom-up level order traversal of its nodes' values_. (i.e., from left to right, level by level from leaf to root).

**Example 1:**

![](https://assets.leetcode.com/uploads/2021/02/19/tree1.jpg)

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

**Output:** [[15,7],[9,20],[3]] 

**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`