TREE TRAVERSALS

In-order
 - Traverse the left subtree
 - Visit the root (i.e., print, etc.)
 - Traverse the right subtree

Pre-order
 - Visit the root (i.e., print, etc.)
 - Traverse the left subtree
 - Traverse the right subtree

Post-order
 - Traverse the left subtree
 - Traverse the right subtree
 - Visit the root (i.e., print, etc.)


Iterative in-order traversal of a BST (1-2-3-4-5-6-7)

Push the 4 onto the stack
Go to the left subtree
Push the 2 onto the stack
Go to the left subtree
Print the 1    # in code, we will push the 1 and the pop it in the next iteration
Pop -> now we're at the 2
Print the 2
Push the 3 (right subtree of 2)
Pop -> now we're at the 3
Print the 3    # in code, we will push the 3 and the pop it in the next iteration
Pop -> now we're at the 4
Print the 4    # in code, we will push the 4 and the pop it in the next iteration
Push the 6 (right subtree of 4)
Pop -> now we're at the 6
Go to the left subtree of 6
Print the 5    # in code, we will push the 5 and the pop it in the next iteration
Pop -> now we're at the 6 again
Print the 6
Push the 7 (right subtree of 6)
Print the 7    # in code, we will push the 7 and the pop it in the next iteration