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?