Flatten Binary Tree to Linked List

flatten_binary_tree.py

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

Example:
Input:
   1
  / \
 2   5
 /\   \
3  4   6
Output:
1
 \
  2
   \
    3
     \
      4
       \
        5
         \
          6
"""
class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

def flatten(root):
    """
    :type root: TreeNode
    :rtype: void Do not return anything, modify root in-place instead.
    """
    stack = [root]
    prev = None
    while stack:
        cur = stack.pop()
        if prev:
            prev.right = cur
            prev.left = None
        prev = cur
        if cur.right:
            stack.append(cur.right)
        if cur.left:
            stack.append(cur.left)