156. Binary Tree Upside Down (Medium) - TengnanYao/daily_leetcode GitHub Wiki

# 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:
            return None
        stack = []
        while root:
            stack.append(root)
            root = root.left
        root = node = stack.pop()
        while stack:
            right = stack.pop()
            node.right = right
            node.left = right.right
            right.right = None
            right.left = None
            node = node.right
        return root