# 1666. Change the Root of a Binary Tree

<https://leetcode.com/problems/change-the-root-of-a-binary-tree>

## Description

Given the `root` of a binary tree and a `leaf` node, reroot the tree so that the `leaf` is the new root.

You can reroot the tree with the following steps for each node `cur` on the path **starting from the** `leaf` up to the `root`​​​ **excluding the root**:

1. If `cur` has a left child, then that child becomes `cur`'s right child.
2. `cur`'s original parent becomes `cur`'s left child. Note that in this process the original parent's pointer to `cur` becomes `null`, making it have at most one child.

Return *the new root* *of the rerooted tree.*

**Note:** Ensure that your solution sets the `Node.parent` pointers correctly after rerooting or you will receive "Wrong Answer".

**Example 1:**

![](https://assets.leetcode.com/uploads/2020/11/24/fliptree.png)

```
**Input:** root = [3,5,1,6,2,0,8,null,null,7,4], leaf = 7
**Output:** [7,2,null,5,4,3,6,null,null,null,1,null,null,0,8]
```

**Example 2:**

```
**Input:** root = [3,5,1,6,2,0,8,null,null,7,4], leaf = 0
**Output:** [0,1,null,3,8,5,null,null,null,6,2,null,null,7,4]
```

**Constraints:**

* The number of nodes in the tree is in the range `[2, 100]`.
* `-109 <= Node.val <= 109`
* All `Node.val` are **unique**.
* `leaf` exist in the tree.

## ac

```java
```