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# 0107. Binary Tree Level Order Traversal II

<https://leetcode.com/problems/binary-tree-level-order-traversal-ii>

## Description

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`

## ac

```java
```