156 Binary Tree Upside Down

Given the root of a binary tree, turn the tree upside down and return the new root.

You can turn a binary tree upside down with the following steps:

The original left child becomes the new root. The original root becomes the new right child. The original right child becomes the new left child.

  X           X           Y
 / \    =>   /      =>   / \
Y   Z       Y - Z       Z   X

The mentioned steps are done level by level. It is guaranteed that every right node has a sibling (a left node with the same parent) and has no children.

Example 1:

Input: root = [1,2,3,4,5]
Output: [4,5,2,null,null,3,1]

Example 2:

Input: root = []
Output: []

Example 3:

Input: root = [1]
Output: [1]

Constraints:

• The number of nodes in the tree will be in the range [0, 10].
• 1 <= Node.val <= 10
• Every right node in the tree has a sibling (a left node that shares the same parent).
• Every right node in the tree has no children.

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def upsideDownBinaryTree(
        self,
        root: Optional[TreeNode]
    ) -> Optional[TreeNode]:
        if not root or (not root.left and not root.right):
            return root
        new_root = self.upsideDownBinaryTree(root.left)
        root.left.left = root.right
        root.left.right = root
        root.left = root.right = None
        return new_root