Binary Trees

Why Care?

A binary tree is just a k-ary tree with k = 2. Because k is only 2,

Special Traversals

Like all oriented trees, breadth-first and depth-first traversals exist for binary trees, but there are others:

bintraverse.png

  • Preorder: C F G A D E H B I J K
  • Inorder: G F D A E C I B K J H
  • Postorder: G D E A F I K J B H C

Binary Trees as Strings

Three simple rules:

So for example:

How Many Binary Trees Are There?

There are five distinct shapes of binary trees with three nodes:

allthreenodebinarytrees.png

But how many are there for n nodes?

Let C(n) be the number of distinct binary trees with n nodes. This is equal to the number of trees that have a root, a left subtree with j nodes, and a right subtree of (n-1)-j nodes, for each j. That is,

    C(n) = C(0)C(n-1) + C(1)C(n-2) + ... + C(n-1)C(0)

which is

catalan1.png

The first few terms:

    C(0) = 1
    C(1) = C(0)C(0) = 1
    C(2) = C(0)C(1) + C(1)C(0) = 2
    C(3) = C(0)C(2) + C(1)C(1) + C(2)C(0) = 5
    C(4) = C(0)C(3) + C(1)C(2) + C(2)C(1) + C(3)C(0) = 14

You can prove

catalan2.png

Here's the number of 8-node binary trees:

           1   16!    16×15×14×13×12×11×10
    C(8) = - × ---- = -------------------- = 13×11×10 = 1430
           9   8!8!      8×7×6×5×4×3×2×1

Also see Wikipedia's article on the Catalan Numbers.

An Implementation of Binary Trees

As is common with trees (though quite unlike lists), node classes are visible to clients. Here is a simple binary tree node interface, again using the visitor pattern for traversals.

BinaryTreeNode.java
/**
 * A simple interface for binary trees. An empty binary tree is represented with
 * the value null; a non-empty tree by its root node.
 */
public interface BinaryTreeNode<E> {

    /**
     * Returns the data stored in this node.
     */
    E getData();

    /**
     * Modifies the data stored in this node.
     */
    void setData(E data);

    /**
     * Returns the parent of this node, or null if this node is a root.
     */
    BinaryTreeNode<E> getParent();

    /**
     * Returns the left child of this node, or null if it does not have one.
     */
    BinaryTreeNode<E> getLeft();

    /**
     * Removes child from its current parent and inserts it as the left child of
     * this node. If this node already has a left child it is removed.
     *
     * @exception IllegalArgumentException if the child is an ancestor of this node,
     *                                     since that would make a cycle in the
     *                                     tree.
     */
    void setLeft(BinaryTreeNode<E> child);

    /**
     * Returns the right child of this node, or null if it does not have one.
     */
    BinaryTreeNode<E> getRight();

    /**
     * Removes child from its current parent and inserts it as the right child of
     * this node. If this node already has a right child it is removed.
     * 
     * @exception IllegalArgumentException if the child is an ancestor of this node,
     *                                     since that would make a cycle in the
     *                                     tree.
     */
    void setRight(BinaryTreeNode<E> child);

    /**
     * Removes this node, and all its descendants, from whatever tree it is in. Does
     * nothing if this node is a root.
     */
    void removeFromParent();

    /**
     * Visits the nodes in this tree in preorder.
     */
    void traversePreorder(Visitor visitor);

    /**
     * Visits the nodes in this tree in postorder.
     */
    void traversePostorder(Visitor visitor);

    /**
     * Visits the nodes in this tree in inorder.
     */
    void traverseInorder(Visitor visitor);

    /**
     * Simple visitor interface.
     */
    public interface Visitor {
        <E> void visit(BinaryTreeNode<E> node);
    }
}

We can implement this interface with links

LinkedBinaryTreeNode.java
/**
 * An implementation of the BinaryTreeNode interface in which each node stores
 * direct links to its left child, its right child, and its parent.
 *
 * <p>
 * LinkedBinaryTreeNode objects are pretty mean: if one tries to mix them up
 * with different kinds of binary tree nodes, and exception may be thrown.
 * </p>
 */
public class LinkedBinaryTreeNode<E> implements BinaryTreeNode<E> {
    protected E data;
    protected LinkedBinaryTreeNode<E> parent;
    protected LinkedBinaryTreeNode<E> left;
    protected LinkedBinaryTreeNode<E> right;

    /**
     * Constructs a node as the root of its own one-element tree. This is the only
     * public constructor. The only trees that clients can make directly are simple
     * one-element trees.
     */
    public LinkedBinaryTreeNode(E data) {
        this.data = data;
    }

    /**
     * Returns the data stored in this node.
     */
    public E getData() {
        return data;
    }

    /**
     * Modifies the data stored in this node.
     */
    public void setData(E data) {
        this.data = data;
    }

    /**
     * Returns the parent of this node, or null if this node is a root.
     */
    public BinaryTreeNode<E> getParent() {
        return parent;
    }

    /**
     * Returns the left child of this node, or null if it does not have one.
     */
    public BinaryTreeNode<E> getLeft() {
        return left;
    }

    /**
     * Removes child from its current parent and inserts it as the left child of
     * this node. If this node already has a left child it is removed.
     * 
     * @exception IllegalArgumentException if the child is an ancestor of this node,
     *                                     since that would make a cycle in the
     *                                     tree.
     */
    public void setLeft(BinaryTreeNode<E> child) {
        // Ensure the child is not an ancestor.
        for (LinkedBinaryTreeNode<E> n = this; n != null; n = n.parent) {
            if (n == child) {
                throw new IllegalArgumentException();
            }
        }

        // Ensure that the child is an instance of LinkedBinaryTreeNode.
        LinkedBinaryTreeNode<E> childNode = (LinkedBinaryTreeNode<E>) child;

        // Break old links, then reconnect properly.
        if (this.left != null) {
            left.parent = null;
        }
        if (childNode != null) {
            childNode.removeFromParent();
            childNode.parent = this;
        }
        this.left = childNode;
    }

    /**
     * Returns the right child of this node, or null if it does not have one.
     */
    public BinaryTreeNode<E> getRight() {
        return right;
    }

    /**
     * Removes child from its current parent and inserts it as the right child of
     * this node. If this node already has a right child it is removed.
     * 
     * @exception IllegalArgumentException if the child is an ancestor of this node,
     *                                     since that would make a cycle in the
     *                                     tree.
     */
    public void setRight(BinaryTreeNode<E> child) {
        // Ensure the child is not an ancestor.
        for (LinkedBinaryTreeNode<E> n = this; n != null; n = n.parent) {
            if (n == child) {
                throw new IllegalArgumentException();
            }
        }

        // Ensure that the child is an instance of LinkedBinaryTreeNode.
        LinkedBinaryTreeNode<E> childNode = (LinkedBinaryTreeNode<E>) child;

        // Break old links, then reconnect properly.
        if (right != null) {
            right.parent = null;
        }
        if (childNode != null) {
            childNode.removeFromParent();
            childNode.parent = this;
        }
        this.right = childNode;
    }

    /**
     * Removes this node, and all its descendants, from whatever tree it is in. Does
     * nothing if this node is a root.
     */
    public void removeFromParent() {
        if (parent != null) {
            if (parent.left == this) {
                parent.left = null;
            } else if (parent.right == this) {
                parent.right = null;
            }
            this.parent = null;
        }
    }

    /**
     * Visits the nodes in this tree in preorder.
     */
    public void traversePreorder(BinaryTreeNode.Visitor visitor) {
        visitor.visit(this);
        if (left != null)
            left.traversePreorder(visitor);
        if (right != null)
            right.traversePreorder(visitor);
    }

    /**
     * Visits the nodes in this tree in postorder.
     */
    public void traversePostorder(Visitor visitor) {
        if (left != null)
            left.traversePostorder(visitor);
        if (right != null)
            right.traversePostorder(visitor);
        visitor.visit(this);
    }

    /**
     * Visits the nodes in this tree in inorder.
     */
    public void traverseInorder(Visitor visitor) {
        if (left != null)
            left.traverseInorder(visitor);
        visitor.visit(this);
        if (right != null)
            right.traverseInorder(visitor);
    }
}

For fun, here is a panel class that can draw a binary tree.

BinaryTreePanel.java
import java.awt.Color;
import java.awt.FontMetrics;
import java.awt.Graphics;
import java.awt.Point;
import java.awt.Rectangle;
import java.lang.reflect.Field;
import java.util.HashMap;
import java.util.Map;

import javax.swing.JPanel;

/**
 * A panel that maintains a picture of a binary tree.
 */
public class BinaryTreePanel extends JPanel {
    private BinaryTreeNode<?> tree;
    private int gridwidth;
    private int gridheight;

    /**
     * Stores the pixel values for each node in the tree.
     */
    private Map<BinaryTreeNode<?>, Point> coordinates = new HashMap<BinaryTreeNode<?>, Point>();

    /**
     * Constructs a panel, saving the tree and drawing parameters.
     */
    public BinaryTreePanel(BinaryTreeNode<?> tree, int gridwidth, int gridheight) {
        this.tree = tree;
        this.gridwidth = gridwidth;
        this.gridheight = gridheight;
    }

    /**
     * Changes the tree rendered by this panel.
     */
    public void setTree(BinaryTreeNode<?> root) {
        tree = root;
        repaint();
    }

    /**
     * Draws the tree in the panel. First it computes the coordinates of all the
     * nodes with an inorder traversal, then draws them with a postorder traversal.
     */
    public void paintComponent(final Graphics g) {
        super.paintComponent(g);

        if (tree == null) {
            return;
        }

        tree.traverseInorder(new BinaryTreeNode.Visitor() {
            private int x = gridwidth;

            public void visit(BinaryTreeNode node) {
                coordinates.put(node, new Point(x, gridheight * (depth(node) + 1)));
                x += gridwidth;
            }
        });

        tree.traversePostorder(new BinaryTreeNode.Visitor() {
            public void visit(BinaryTreeNode node) {
                String data = node.getData().toString();
                Point center = (Point) coordinates.get(node);
                if (node.getParent() != null) {
                    Point parentPoint = (Point) coordinates.get(node.getParent());
                    g.setColor(Color.black);
                    g.drawLine(center.x, center.y, parentPoint.x, parentPoint.y);
                }
                FontMetrics fm = g.getFontMetrics();
                Rectangle r = fm.getStringBounds(data, g).getBounds();
                r.setLocation(center.x - r.width / 2, center.y - r.height / 2);
                Color color = getNodeColor(node);
                Color textColor = (color.getRed() + color.getBlue() + color.getGreen() < 382) ? Color.white
                        : Color.black;
                g.setColor(color);
                g.fillRect(r.x - 2, r.y - 2, r.width + 4, r.height + 4);
                g.setColor(textColor);
                g.drawString(data, r.x, r.y + r.height);
            }
        });
    }

    /**
     * Returns a color for the node. If the node is of a class with a field called
     * "color", and that field currently contains a non-null value, then that value
     * is returned. Otherwise a default color of yellow is returned.
     */
    Color getNodeColor(BinaryTreeNode<?> node) {
        try {
            Field field = node.getClass().getDeclaredField("color");
            return (Color) field.get(node);
        } catch (Exception e) {
            return Color.yellow;
        }
    }

    private int depth(BinaryTreeNode<?> node) {
        return (node.getParent() == null) ? 0 : 1 + depth(node.getParent());
    }
}