使用PushbackIterator合并列表？

Question

I have to use a PushbackIterator which uses a pushback(E) method.

So if the value in aIter is less than the value in bIter , aIter is returned, while the value in bIter is put into the pushback(E) method, and vice versa.

If the original lists are

(1, 4, 5) and (2, 3, 6)

then the result list would be

(1, 2, 3, 4, 5, 6)

.

I believe that I need to determine if an iterator has no values left. So if bIter has no more elements, I get an element from aIter and add it to the result list.

How would I determine if an iterator has no more values?

public static<E extends Comparable<E>> List<E> mergeSortedLists(List<E> a, List<E> b) {
    List<E> result = new ArrayList<E>();

    PushbackIterator<E> aIter = new PushbackIterator<E>(a.iterator());
    PushbackIterator<E> bIter = new PushbackIterator<E>(b.iterator());

    while (aIter.hasNext() && bIter.hasNext()) {
        if (aIter.next().compareTo(bIter.next()) < 0) {
            result.add(aIter.next());
            aIter.pushback(bIter.next());
        } 
        if (aIter.next().compareTo(bIter.next()) > 0)
            result.add(bIter.next());
            bIter.pushback(aIter.next());
        }

    return result;
}

Answer 1

The while terminates if one of the iterators has no more elements, thus afterwards you can just add the remaining elements of the iterators:

while (aIter.hasNext()) {
    result.add(aIter.next());
}
while (bIter.hasNext()) {
    result.add(bIter.next());
}

Btw, your code has some flaws since you call .next() multiple times before adding an element. Because of this you will skip elements. Try this:

public static<E extends Comparable<E>> List<E> mergeSortedLists(List<E> a, List<E> b) {
    List<E> result = new ArrayList<E>();

    PushbackIterator<E> aIter = new PushbackIterator<E>(a.iterator());
    PushbackIterator<E> bIter = new PushbackIterator<E>(b.iterator());

    while (aIter.hasNext() && bIter.hasNext()) {
        E aElem = aIter.next();
        E bElem = bIter.next();
        if (aElem.compareTo(bElem) <= 0) {
            result.add(aElem);
            bIter.pushback(bElem);
        } else {
            result.add(bElem);
            aIter.pushback(aElem);
        }
    }

    while (aIter.hasNext()) {
        result.add(aIter.next());
    }
    while (bIter.hasNext()) {
        result.add(bIter.next());
    }

    return result;
}