Leetcode: Binary Tree Upside Down

## https://code.dennyzhang.com/binary-tree-upside-down
## Basic Ideas: In-order
##
## Complexity: Time O(n), Space O(h). h = height of the tree
# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def upsideDownBinaryTree(self, root):
        """
        :type root: TreeNode
        :rtype: TreeNode
        """
        if root is None: return None
        stack = []
        p = root
        while p:
            stack.append(p)
            p = p.left
        newRoot = stack.pop(-1)
        q = newRoot
        while len(stack) != 0:
            pre_parent = stack.pop(-1)
            q.left = pre_parent.right
            q.right = pre_parent
            q = pre_parent
        # configure the last node as a leaf
        q.left, q.right = None, None
        return newRoot