Question

The C++ STL class std::map implements O(log(n)) look-up using a binary tree. But with trees, it’s not immediately obvious how an iterator would work. What does the ++ operator actually mean in a tree structure? Whereas the concept of “next element” has an obvious implementation in an array, for me it’s not so obvious in a tree. How would one implement a tree iterator?

Answer 1

For an inorder traversal (probably works for others too), if you have a parent-pointer in your nodes you can do a non-recursive traversal. It should be possible to just store two pointers in your iterator: you need an indication of where you are, and you’ll probably (I’m not doing the research now) need something like a “previous” pointer so you can figure out your current movement direction (i.e. do I need to go into the left subtree, or did I just come back from it).

“Previous” will probably be something like “parent”, if we’ve just entered the node; “left” if we’re coming back from the left subtree, “right” if we are coming back from the right subtree, and “self” if the last node we returned was our own.