# 0099. Recover Binary Search Tree

<https://leetcode.com/problems/recover-binary-search-tree>

## Description

You are given the `root` of a binary search tree (BST), where the values of **exactly** two nodes of the tree were swapped by mistake. *Recover the tree without changing its structure*.

**Example 1:**

![](https://assets.leetcode.com/uploads/2020/10/28/recover1.jpg)

```
**Input:** root = [1,3,null,null,2]
**Output:** [3,1,null,null,2]
**Explanation:** 3 cannot be a left child of 1 because 3 > 1. Swapping 1 and 3 makes the BST valid.
```

**Example 2:**

![](https://assets.leetcode.com/uploads/2020/10/28/recover2.jpg)

```
**Input:** root = [3,1,4,null,null,2]
**Output:** [2,1,4,null,null,3]
**Explanation:** 2 cannot be in the right subtree of 3 because 2 < 3. Swapping 2 and 3 makes the BST valid.
```

**Constraints:**

* The number of nodes in the tree is in the range `[2, 1000]`.
* `-231 <= Node.val <= 231 - 1`

**Follow up:** A solution using `O(n)` space is pretty straight-forward. Could you devise a constant `O(1)` space solution?

## ac

```java
```