BST Iterators: Overview

The Problem: Iterator-defined traversal should encounter elements in order

       40                   20                   50
      /  \                 /  \                 /  \
   20      60            10    50             30   60
  /  \    /  \                /  \           /  \    \
 10  30  50  70              40  70         20  40   70
                            /    /         /
                           30   60        10

Output should be linear regardless of internal structure: 10 20 30 40 50 60 70

Possible Strategies:

1. Navigation: (aka "tree monkey") Add a parent pointer to each node, and use the three pointers to navigate ("climb") around in the tree
2. Threading: Use the null pointers in a tree to store "next node" information when it cannot be computed by descending in the tree
3. ADT Control: Use a Stack or Queue to keep track of where an iterator is in a tree
4. Graph Algorithms: Use a standard Graph Depth-First Iterator or Breadth-First Iterator, applied specifically to the tree considered as a directed graph.