Want to save binary tree to disk for "20 questions" game

Question

In short, I'd like to learn/develop an elegant method to save a binary tree to disk (a general tree, not necessarily a BST). Here is the description of my problem:

I'm implementing a game of "20-questions". I've written a binary tree whose internal nodes are questions and leaves are answers. The left child of a node is the path you'd follow if somebody answered "yes" to your current question, while the right child is a "no" answer. Note this is not a binary search tree, just a binary tree whose left child is "yes" and right is "no".

The program adds a node to a tree if it encounters a leaf that is null by asking the user to distinguish her answer from the one the computer was thinking of.

This is neat, because the tree builds itself up as the user plays. What's not neat is that I don't have a good way of saving the tree to disk.

I've thought about saving the tree as an array representation (for node i, left child is 2i+1, and 2i+2 right, (i-1)/2 for parent), but it's not clean and I end up with a lot of wasted space.

Any ideas for an elegant solution to saving a sparse binary tree to disk?

Answer 1

You can store it recursively:

 void encodeState(OutputStream out,Node n) {
        if(n==null) {
            out.write("[null]");
        } else {
           out.write("{");
           out.write(n.nodeDetails());
           encodeState(out, n.yesNode());
           encodeState(out, n.noNode());
           out.write("}");
        }
  }

Devise your own less texty output format. I'm sure I don't need to describe the method to read the resulting output.

This is depth-first traversal. Breadth-first works too.