title: Iterating over a Binary Tree with O(1) Auxiliary Space
body: <p>Is it possible to iterate over a binary tree in O(1) auxiliary space (w/o using a stack, queue, etc.), or has this been proven impossible?  If it is possible, how can it be done?</p>
  
  <p>Edit:  The responses I've gotten about this being possible if there are pointers to parent nodes are interesting and I didn't know that this could be done, but depending in how you look at it, that can be O(n) auxiliary space.  Furthermore, in my practical use case, there are no pointers to parent nodes.  From now on, please assume this when answering.</p>