LeetCode (Python): 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

Solution:

Stack Solution:

# Definition for a  binary tree node
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    # @param root, a tree node
    # @return nothing, do it in place
    def flatten(self, root):
       if root == None:
           return
       stack = []
       stack.append(root)
       while len(stack) > 0:
           actual = stack.pop();
           if actual.right != None:
               stack.append(actual.right)
           if actual.left != None:
               stack.append(actual.left)
           if len(stack) > 0:
               actual.right = stack[len(stack)-1]
           actual.left = None

Inplace Solution:

# Definition for a  binary tree node
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    # @param root, a tree node
    # @return nothing, do it in place
    def flatten(self, root):
        while root != None:
            if root.left != None:
                pre = root.left
                while pre.right != None:
                    pre = pre.right
                pre.right = root.right
                root.right = root.left
                root.left = None
            root = root.right