Draw the process on a paper and you will get the solution easily.
The upside down is actually done on the left path of the tree.
# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution:
def upsideDownBinaryTree(self, root: TreeNode) -> TreeNode:
if root is None:
return None
new_root = root
while new_root.left:
new_root = new_root.left
self._helper(root, None, None)
return new_root
def _helper(self, node, parent, right):
if node is None:
return
self._helper(node.left, node, node.right)
node.left, node.right = right, parent