## Description #

Difficulty: **中等**

Related Topics: Tree, Breadth-First Search, Binary Tree

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:**

```
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`

## Solution #

Language: **Go**

IOI