[LC] 114. Flatten Binary Tree to Linked List

 1 # Definition for a binary tree node.
 2 # class TreeNode(object):
 3 #     def __init__(self, x):
 4 #         self.val = x
 5 #         self.left = None
 6 #         self.right = None
 7 
 8 class Solution(object):
 9     def flatten(self, root):
10         """
11         :type root: TreeNode
12         :rtype: None Do not return anything, modify root in-place instead.
13         """
14         self.prev = None
15         def dfs(root):
16             if root is None:
17                 return None
18             # reverse preOrder traveral
19             # use pre_node to track the previous node to connect as current right
20             dfs(root.right)
21             dfs(root.left)
22             root.right = self.prev
23             root.left = None
24             self.prev = root
25         dfs(root)