public class Solution
extends Object
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:**
**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)?