> For the complete documentation index, see [llms.txt](https://mayanktyagi3111.gitbook.io/interview-prep/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://mayanktyagi3111.gitbook.io/interview-prep/trees/recover-binary-search-tree.md).

# Recover Binary Search Tree

Two elements of a binary search tree (BST) are swapped by mistake.

Recover the tree without changing its structure.

**Example 1:**

```
Input: [1,3,null,null,2]

   1
  /
 3
  \
   2

Output: [3,1,null,null,2]

   3
  /
 1
  \
   2
```

**Example 2:**

```
Input: [3,1,4,null,null,2]

  3
 / \
1   4
   /
  2

Output: [2,1,4,null,null,3]

  2
 / \
1   4
   /
  3
```

**Follow up:**

* A solution using O(*n*) space is pretty straight forward.
* Could you devise a constant space solution?

```java
class Solution {
    public void recoverTree(TreeNode root) {
        TreeNode pre = null;
        TreeNode first = null, second = null;
        // Morris Traversal
        // IN ORDER traversal using Morris
        while (root != null) {
            if (root.left != null) {
                // connect threading for root
                TreeNode temp = root.left;
                while (temp.right != null && temp.right != root)
                    temp = temp.right;
                // the threading already exists then just deconstruct the thread
                if (temp.right != null) {
                    temp.right = null;

                    if (pre != null && pre.val > root.val) {
                        if (first == null)
                            first = pre;
                        second = root;
                    }
                    pre = root;
                    root = root.right;
                } else {
                    // construct the threading
                    temp.right = root;
                    root = root.left;
                }
            } else {
                if (pre != null && pre.val > root.val) {
                    if (first == null)
                        first = pre;
                    second = root;
                }
                pre = root;
                root = root.right;
            }
        }
        // swap two node values;
        if (first != null && second != null) {
            int t = first.val;
            first.val = second.val;
            second.val = t;
        }
    }
}
```


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://mayanktyagi3111.gitbook.io/interview-prep/trees/recover-binary-search-tree.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.