114\. Flatten Binary Tree to Linked List

Medium

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**](https://en.wikipedia.org/wiki/Tree_traversal#Pre-order,_NLR) of the binary tree.

**Example 1:**

![](https://assets.leetcode.com/uploads/2021/01/14/flaten.jpg)

**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)?