Source: cirosantilli/iterative-in-order

= 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: https://www.geeksforgeeks.org/inorder-tree-traversal-without-recursion/

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