Flatten Binary Tree to Linked List

Given a binary tree, flatten it to a linked list in-place.

For example, Given

     1
    / \
   2   5
  / \   \
 3   4   6

The flattened tree should look like: 1 \ 2 \ 3 \ 4 \ 5 \ 6

URL: https://leetcode.com/problems/flatten-binary-tree-to-linked-list/

# 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 flatten(self, root):
        """
        :type root: TreeNode
        :rtype: void Do not return anything, modify root in-place instead.
        """
        if root == None:
            return root

        stack = []
        current = root

        while((stack != []) or (current != None)):

            if current.right != None:
                stack.append(current.right)

            if current.left != None:
                current.right = current.left
                current.left = None
            else:
                if stack != []:
                    temp = stack.pop()
                    current.right = temp

            current = current.right