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.

• The root of a binary tree.

Input: root = [1, 2, 5, 3, 4, null, 6]
Output: [1, null, 2, null, 3, null, 4, null, 5, null, 6]