Flatten Binary Tree 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 where the right child pointer points to the next node in the list and the left child pointer is always null.
The "linked list" should be in the same order as a pre-order traversal of the binary tree.

Follow up: Can you flatten the tree in-place with O(1) extra space?

Example 1

        1
       / \
      2   5      ->      [1] -> [2] -> [3] -> [4] -> [5] -> [6] -> null
     / \   \
    3   4   6

Inputroot = [1,2,5,3,4,null,6]

Output[1,null,2,null,3,null,4,null,5,null,6]

The tree is flattened in preorder so every node appears along the right pointers and every left pointer is null.

Example 2

null
---
null

Inputroot = []

Output[]

An empty tree remains empty after flattening.

Constraints