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.
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.
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].
-2^31 <= Node.val <= 2^31 - 1
Follow up: A solution using O(n) space is pretty straight-forward.
Could you devise a constant O(1) space solution?