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:

**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:

**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?

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