Problem - Recover BST

Two nodes of a BST were swapped by mistake. Recover the tree without changing its structure. Example: In $[1,3,null,null,2]$, nodes $1$ and $3$ were swapped.

Correct tree is $[3,1,null,null,2]$. The BST property is violated at two positions. You need to identify which two nodes were swapped and fix them.

Follow-up: Can you solve it using $O(1)$ space (excluding recursion stack)? Constraints: up to $1000$ nodes.