/** Scheme-like pairs that can be used to form a list of 
 *  integers. */
public class IntList {
    public int head;
    public IntList tail;

    /** A List with head and tail. */
    public IntList (int head, IntList tail) {
        this.head = head; this.tail = tail;
    }

    /** A List with null tail, and head = 0. */
    public IntList () { this (0, null); }
    // NOTE: public IntList () { }  would also work.

    /** A new IntList containing the ints in ARGS. */
    public static IntList list (Integer ... args) {
        IntList sentinel = new IntList (0, null);

        IntList p;
        p = sentinel;
        for (Integer x : args)
            p = p.tail = new IntList (x, null);
        return sentinel.tail;
    }

    /** A new IntList containing the ints A. */
    public static IntList list (int[] A) {
        IntList sentinel = new IntList (0, null);

        IntList p;
        p = sentinel;
        for (int x : A)
            p = p.tail = new IntList (x, null);
        return sentinel.tail;
    }


    /** True iff X is an IntList or int[] containing the same sequence of ints
     *  as THIS. */
    public boolean equals (Object x) {
        if (x instanceof IntList) {
            IntList L = (IntList) x;
            IntList p;
            for (p = this; p != null && L != null; p = p.tail, L = L.tail) {
                if (p.head != L.head)
                    return false;
            }
            if (p == null && L == null)
                return true;
        } else if (x instanceof int[]) {
            int[] A = (int[]) x;
            IntList p;
            int i;
            for (i = 0, p = this; i < A.length && p != null; 
                 i += 1, p = p.tail) {
                if (A[i] != p.head)
                    return false;
            }
            if (i == A.length && p == null)
                return true;
        }
        return false;
    }

    /** Convert THIS into a readable String. */
    public String toString () {
        StringBuffer b = new StringBuffer ();
        b.append ("[");
        for (IntList L = this; L != null; L = L.tail) 
            b.append (" " + L.head); 
        b.append ("]");
        return b.toString ();
    }

}