I started with some basic terminology. I drew a crude picture of a tree and pointed out that we refer to the base of the tree as its root and we talk about branches of the tree with leaves on the branches. Computer scientists view the world upside down, so we draw a tree with the root a the top and the leaves at the bottom. For example, we might draw this tree:
12
/ \
/ \
18 7
/ \
/ \
4 13
Just as with linked lists, we refer to each different value as a node. At the
top of this tree we have the root node storing the value 12. We would refer to
the nodes with 12 and 7 as branch nodes of the tree because they have values
stored under them. The nodes with values 18, 4 and 13 are leaf nodes of the
tree because each one has nothing under it.Another set of terms we use is parent and child. The root node is the ultimate ancestor of every other node. It is the parent of the nodes 18 and 7. Similarly, we'd say that the parents of 18 and 7 are the root node 12. We also sometimes refer to 18 and 7 as siblings.
I then gave a recursive definition of a tree. I said that a tree is either:
+-----------+
| root node |
+-----------+
/ \
/ \
/ \
/\ /\
/ \ / \
/ \ / \
/ left \ / right\
/ subtree\ / subtree\
+----------+ +----------+
That will be a useful way to think about trees as we write binary tree code.
Using our recursive definition, we discussed how you could form various kinds
of trees. The simplest kind of tree is an empty tree, which can't really be
drawn because it's empty.Once you have an empty tree, you can use the second part of the definition to make a new kind of tree that is composed of a root node with left and right subtrees that are both empty. In other words, that would be a single leaf node, which we could represent with a star:
*
Now that we have this as an option, we can use our recursive rule to say that a
tree could be a root node with a left tree that is empty and a right tree that
is a leaf:
*
\
*
Or we can have an empty right and a leaf to the left:
*
/
*
Or we could have leaf on either side:
*
/ \
* *
These now become possibilities to use for our recursive definition, allowing us
to construct even more kinds of trees, as in:
* * * *
/ \ / \ / \ / \
* * * * * * * *
/ \ / \ / \ \ / \ \
* * * * * * * * * *
Then I showed people the node class we'll be using for a simple binary tree of
ints:
public class IntTreeNode {
public int data;
public IntTreeNode left;
public IntTreeNode right;
public IntTreeNode(int data) {
this(data, null, null);
}
public IntTreeNode(int data, IntTreeNode left, IntTreeNode right) {
this.data = data;
this.left = left;
this.right = right;
}
}
As with our linked list node, this node is very simple in that it has just some
public data fields and a few constructors. The node has a data field of type
int and two links of type IntTreeNode for the left and right subtrees. The
first constructor constructs a leaf node (using null for left and right). The
second constructor would be appropriate for a branch node where you want to
specify the left and right subtrees.This node class is "messy" in the sense that it is not well encapsulated. We did something similar with the linked list nodes and I made the analogy that they are like the cans of paint that a contractor might use in painting your house. We want a cleaner interface for dealing with a client, so I mentioned that we'll have a second object for storing a tree. Any external client will deal with the IntTree object and won't ever see these tree node objects.
We need only one data field in the tree class: a reference to the root of the tree:
public class IntTree {
private IntTreeNode overallRoot;
...
}
I pointed out that I'm purposely using the name "overallRoot" to distinguish
this root from all of the other roots. There is only one overall root. But
each subtree is itself the root of a tree and in our recursive methods we'll
often use the parameter name "root" as a way to indicate that it can be any of
the roots.I then spent time discussing the idea of tree traversals. The idea is to "traverse" the tree in such a way that you visit each node exactly once. There are many different ways to do this. We generally prefer recursive approaches, so we want to traverse the entire left subtree without dealing with anything from the right and in a separate operation, traverse the entire right subtree without dealing with anything from the left. That leads to the classic binary tree traversals. We have a Western bias that we traverse the left subtree before the right subtree. The question becomes, where do you deal with the root of the tree?
There are three possible answers you might give. You can process the root before you traverse either subtree, after you traverse both subtrees or in between traversing the two subtrees. These three approaches are known as preorder, inorder and postorder traversals.
+------------------+
+-----------<-------------+ process the root +------------->-----------+
| +--------+---------+ |
| | |
| | |
V +-----------------------+ V +------------------------+ V
pre | traverse left subtree | in | traverse right subtree | post
+-----------------------+ +------------------------+
For example, given the following tree:
+---+
| 2 |
+---+
/ \
/ \
+---+ +---+
| 0 | | 3 |
+---+ +---+
/ / \
/ / \
+---+ +---+ +---+
| 7 | | 1 | | 9 |
+---+ +---+ +---+
/ \ / \
/ \ / \
+---+ +---+ +---+ +---+
| 6 | | 5 | | 8 | | 4 |
+---+ +---+ +---+ +---+
The traversals would be as follows:
Then I turned back to our sample IntTreeNode and IntTree classes and talked about how we could write some basic methods for the class. In particular, I wanted to write a public method for the IntTree class that would print the tree values using a preorder traversal. So our public method will look like this:
public void printPreorder() {
...
}
I mentioned that it will almost always be the case when you go to write such a
method that you actually have to write a pair of methods to solve the problem.
The issue is that from the client's point of view, they want to print the
entire tree. But to solve this problem recursively, we need a method that
works on every subtree, not just the overall tree. We need a method that takes
a IntTreeNode as a parameter, so we'll create a private method:
private void printPreorder(IntTreeNode root) {
...
}
We'll start the recursive process by passing it the overall root. We can also
include code to print text at the beginning and end of the line of output we
want to produce:
public void printPreorder() {
System.out.print("preorder:");
printPreorder(overallRoot);
System.out.println();
}
But how do we write the private method? I told people that it's good to go
back to the basic definition of a binary tree. Remember that it is either an
empty tree or it is a root node with left and right subtrees. If it's an empty
tree, then there isn't anything to print. That means we could begin our
private method this way:
private void printPreorder(IntTreeNode root) {
if (root == null)
// do nothing
...
}
But since we have nothing to do in this case, it's better to test the negation
of this:
private void printPreorder(IntTreeNode root) {
if (root != null) {
...
}
}
So what do we do in the case where node is not null? That would mean we have a
root node that has some data in it and we have left and right subtrees that
need to be printed. The way a preorder traversal works is that we handle the
root node first, which means we'd print out the data for the root. We include
a space to separate it from other values on the line of output:
private void printPreorder(IntTreeNode root) {
if (root != null) {
System.out.print(" " + root.data);
...
}
}
What do we do after print the data for this node? We want to print the left
subtree in a preorder manner and then print the right subtree in a preorder
manner. If you're thinking recursively, you'll think, "If only I had a method
to print a subtree in a preorder manner...but I do have such a method...the one
I'm writing." So this becomes:
private void printPreorder(IntTreeNode root) {
if (root != null) {
System.out.print(" " + root.data);
printPreorder(root.left);
printPreorder(root.right);
}
}
That completes the method. I then asked people how to modify the code to print
the tree in an inorder manner and people said to put the print in between the
two recursive calls. What about a postorder traversal? You put the print
after the two recursive calls.I mentioned that the IntTree class has three methods for these three different traversals. It is important to understand why the seemingly small change of moving the print relative to the recursive calls generates such different behavior. That's because each recursive call potentially processes a large amount of data (an entire subtree).
I also pointed out that the sample program has two other interesting recursive methods. The first is called printSideways and prints the tree with indentation to show the structure of the tree. In the sample output it produces the following:
7
3
6
12
1
11
5
10
2
9
4
8
Imagine rotating this 90 degrees in a clockwise fashion (or rotating the page
it is printed on). Then you get:
1
2 3
4 5 6 7
8 9 10 11 12
If you just add lines to this, you'll see the tree:
1
/ \
/ \
2 3
/ \ / \
4 5 6 7
/ \ / \ /
8 9 10 11 12
To do this, I had to use an inorder traversal that goes right-to-left because
in the original non-rotated output, the top lines of the output are for the
right subtree and the bottom of the output is the left subtree. This is easy
to accomplish in the recursive method by switching the order of the recursive
calls on the left and right subtrees.I also pointed out that the constructor builds a very particular binary tree. It builds a tree whose nodes are numbered sequentially as you read it level by level (overall root of 1, next level of 2, 3, next level of 4, 5, 6, 7, and so on). This tree is easy to build because for each node storing the value n, its children store the values 2n and 2n+1. The recursive method that builds the tree has extra parameters that tell it the maximum node number to construct and the value to store at the next subtree's root.