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# 1469. Find All The Lonely Nodes

<https://leetcode.com/problems/find-all-the-lonely-nodes>

## Description

In a binary tree, a **lonely** node is a node that is the only child of its parent node. The root of the tree is not lonely because it does not have a parent node.

Given the `root` of a binary tree, return *an array containing the values of all lonely nodes* in the tree. Return the list **in any order**.

**Example 1:**

![](https://assets.leetcode.com/uploads/2020/06/03/e1.png)

```
**Input:** root = [1,2,3,null,4]
**Output:** [4]
**Explanation:** Light blue node is the only lonely node.
Node 1 is the root and is not lonely.
Nodes 2 and 3 have the same parent and are not lonely.
```

**Example 2:**

![](https://assets.leetcode.com/uploads/2020/06/03/e2.png)

```
**Input:** root = [7,1,4,6,null,5,3,null,null,null,null,null,2]
**Output:** [6,2]
**Explanation:** Light blue nodes are lonely nodes.
Please remember that order doesn't matter, [2,6] is also an acceptable answer.
```

**Example 3:**

![](https://assets.leetcode.com/uploads/2020/06/03/tree.png)

```
**Input:** root = [11,99,88,77,null,null,66,55,null,null,44,33,null,null,22]
**Output:** [77,55,33,66,44,22]
**Explanation:** Nodes 99 and 88 share the same parent. Node 11 is the root.
All other nodes are lonely.
```

**Example 4:**

```
**Input:** root = [197]
**Output:** []
```

**Example 5:**

```
**Input:** root = [31,null,78,null,28]
**Output:** [78,28]
```

**Constraints:**

* The number of nodes in the `tree` is in the range `[1, 1000].`
* Each node's value is between `[1, 10^6]`.

## ac

```java
```