=======================================================
_______________________________
Recursive In-order traversal
 - Traverse the left  (recursive)
 - Visit the root
 - Traverse the right (recursive)

_______________________________
Iterative in-order traversal

"Visit" = do something (e.g., print the value)

Push the root (so that we can remember to visit it and everything in its right subtree.
Go to its left subtree.
Repeat.

Push 4 node
Go down its left subtree. (curr = curr -> left)
Push 2 node
Go down its left subtree. (curr = curr -> left)
Push 1 node
(nothing to do for its left subtree)
Pop gives us the 1 node
    # Whenever we pop, we will "visit" the node we popped, and push its right subtree.
Print "1"
(nothing to do for its right subtree)
Pop gives us the 2 node
    # Whenever we pop, we will "visit" the node we popped, and push its right subtree.
Print "2"
Push the 3 node
(nothing to do for its left subtree)
Pop gives us the 3 node again


TRAVERSALS
_______________________________
Iterative pre-order traversal

Visit the root to get "curr"
Push its right subtree
Push its left subtree
Pop to get a new "curr"
Visit curr
Push its right subtree
Push its left subtree


_______________________________
Iterative post-order traversals

Push right subtree
Push the "curr"
Go to the left subtree  (curr = curr -> left)

If curr is NULL:
    Pop the stack to get a new curr