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An iterator is an object that facilitates the traversal of a data structure
It keeps a ref to the current element
Knows how to move to a different element
Ex. Above are inbuilt C++ forward and reverse iterators. You can also get constant iterators using crbegin()
In C++ the range-based for-loop has no reverse iterator
Building an iterator for a Binary Tree
Let us define a BinaryTree Node class of generalised key value
Similarly, let's define a class BinaryTree which maintains our Binary Tree
Now we we will build a Pre-order iterator for this tree (Node->Left->Right)
We can also set this as the default iterator for our BinaryTree class
Now to finally create a tree, we can do something like the below
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