# 0145. Binary Tree Postorder Traversal

https://leetcode.com/problems/binary-tree-postorder-traversal

## Description

Given the `root` of a binary tree, return _the postorder traversal of its nodes' values_.

**Example 1:**

![](https://assets.leetcode.com/uploads/2020/08/28/pre1.jpg)

```
**Input:** root = [1,null,2,3]
**Output:** [3,2,1]
```

**Example 2:**

```
**Input:** root = []
**Output:** []
```

**Example 3:**

```
**Input:** root = [1]
**Output:** [1]
```

**Example 4:**

![](https://assets.leetcode.com/uploads/2020/08/28/pre3.jpg)

```
**Input:** root = [1,2]
**Output:** [2,1]
```

**Example 5:**

![](https://assets.leetcode.com/uploads/2020/08/28/pre2.jpg)

```
**Input:** root = [1,null,2]
**Output:** [2,1]
```

**Constraints:**

* The number of the nodes in the tree is in the range `[0, 100]`.
* `-100 <= Node.val <= 100`

**Follow up:** Recursive solution is trivial, could you do it iteratively?

## ac

```java
```
