Given a binary tree, flatten it to a linked list in-place.

For example, given the following tree:

1

/ \

2 5

/ \ \

3 4 6

The flattened tree should look like:

1

\

2

\

3

\

4

\

5

\

6

```
# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution:
def flatten(self, root: TreeNode) -> None:
"""
Do not return anything, modify root in-place instead.
"""
node, stack = root, []
while node:
if node.right:
stack.append(node.right)
node.right = node.left
node.left = None
if not node.right and stack:
temp = stack.pop()
node.right = temp
node = node.right
```