Problem - Flatten to Linked List

Given the root of a binary tree, flatten the tree into a "linked list": The linked list should use the same TreeNode class.

The right pointer points to the next node, and left is always null. The list should be in preorder. Example: $[1,2,5,3,4,null,6]$ becomes $1 \to 2 \to 3 \to 4 \to 5 \to 6$.

Do it in-place using $O(1)$ extra space (excluding recursion stack). Constraints: up to $2000$ nodes, values from $-100$ to $100$.