//  Yet another implementation of the Tree ADT in Java.
//  In this implementation, emptyTree() returns null.
//  This eliminates one subclass.  Since only once subclass
//  is left, it is folded into the Tree class, and all of
//  the dynamic methods are folded into the static methods.

public class Tree {

    public static Tree emptyTree () {
        return null;
    }

    public static Tree newTree (int n, Tree t1, Tree t2) {
        return new Tree (n, t1, t2);
    }

    public static boolean isEmpty (Tree t) {
        if (t == null)
            return true;
        else return false;
    }

    public static int     label   (Tree t) { return t.n; }
    public static Tree    left    (Tree t) { return t.t1; }
    public static Tree    right   (Tree t) { return t.t2; }

    public static int     height  (Tree t) {
        if (t == null)
            return 0;
        else return t.height;
    }

    public static int     size    (Tree t) {
        if (t == null)
            return 0;
        else return t.size;
    }

    Tree (int n, Tree t1, Tree t2) {
        this.n = n;
        this.t1 = t1;
        this.t2 = t2;
        this.height = 1 + Math.max (height (t1), height (t2));
        this.size = 1 + size (t1) + size (t2);
    }

    private int n;
    private Tree t1;
    private Tree t2;
    private int height;
    private int size;

}