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.

Example 1:

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

Example 2:

    Input: root = []
    Output: []

Example 3:

    Input: root = [0]
    Output: [0]

Constraints:

- The number of nodes in the tree is in the range `[0, 2000]`.
- `-100 <= Node.val <= 100`

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