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Iterative in-order

This is a bit harder than iterative pre-order: now we have to check if there is a left or right element or not:

(START) push current
if there is left:
- move left
else:
- (ELSE) pop
- visit
- if there is right
  - move right
  - GOTO START
- else:
  - GOTO ELSE

The control flow can be slightly simplified if we allow NULLs: www.geeksforgeeks.org/inorder-tree-traversal-without-recursion/

 Ancestors