# Leetcode 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**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)?

**[Think]**

- need to use recursion to find the deepest left node
- this is reverse pre-order traversal
- create a global TreeNode variable to track current right node

[Second- DFS- using stack]

- adding travel node on current right side, let node.left become null