Well I know there is a post out there from '09, but I'm still not sure how to implement this. I didn't understand the solution that Anton gave here, also it is very old thread: Iterating over a Binary Tree with O(1) Auxiliary Space
Thus, I hope after 8 years we can think of some other solutions, any help is much appreciated.
Note: It was an interview question for my friend, the hint was always knowing where we came from.