# NOTE: this is just a dump of my phone screen.

               1 -> NULL
       2      ->      4 -> NULL
  5   ->   6   ->   7 -> 8 -> NULL
9      ->    10    ->      11 -> NULL


class Node:
  left = None
  right = None
  sibling = None
  
# Fail…
def link_right_sibling(root):
  stack = []
  
  it = root
  while it:
    if stack:
      it.sibling = stack[-1]
    if it.right:
      stack.append(it.right)
    if it.left:
      it = it.left
    elif stack:
      it = stack.pop()
    else:
      break
      
# Fail…
def link_right_sibling(root, right=None):
  if not root:
    return
  root.sibling = right
  link_right_sibling(root.left, root.right)
  link_right_sibling(root.right)
  
def link_right_sibling(root):
  if not root:
    return
    
  li = [root, None]
  it = li.pop(0)
  while it:
    if li:
      it.sibling = li[0]
    if it.left:
      li.append(it.left)
    if it.right:
      li.append(it.right)
    if not li:
      break
    it = li.pop(0)
    if it is None:
      li.append(None)
      it = li.pop(0)