Merging two arraylists without creating third one

Question

Here is one task, i was trying to solve. You must write the function

void merge(ArrayList a, ArrayList b)  {
// code
}

The function recieves two ArrayLists with equal size as input parameters [a1, a2, ..., an], [b1, b2, ..., bn]. The execution result is the 1st ArrayList must contain elements of both lists, and they alternate consistently ([a1, b1, a2, b2, ..., an, bn]) Please read the bold text twice =)

Code must work as efficiently as possible.

Here is my solution

public static void merge(ArrayList a, ArrayList b) {
ArrayList result = new ArrayList();
int i = 0;

Iterator iter1 = a.iterator();
Iterator iter2 = b.iterator();
while ((iter1.hasNext() || iter2.hasNext()) && i < (a.size() + b.size())) {
    if (i % 2 ==0) {
        result.add(iter1.next());
    } else {
        result.add(iter2.next());
    }
    i++;
}

a = result;

}

I know it's not perfect at all. But I can't understand how to merge in the 1st list without creating tmp list.

Thanks in advance for taking part.

You haven't asked us a question. "Here is my assignment; do it for me" doesn't count. — user2357112
– user2357112, Commented Jan 21, 2016 at 21:56
Possible duplicate of Write a method that merges two array lists, alternating elements from both array lists — miken32
– miken32, Commented Jan 21, 2016 at 22:44

John Sensebe · Accepted Answer · 2016-01-21 22:23:48Z

0

Double ArrayList a's size. Set last two elements of a to the last element of the old a and the last element of b. Keep going, backing up each time, until you reach the beginnings of a and b. You have to do it from the rear because otherwise you will write over the original a's values.

edited Jan 21, 2016 at 22:23

answered Jan 21, 2016 at 21:59

John Sensebe

1,3968 silver badges11 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

SergeBud Over a year ago

Thanks, John ! Solution is so simple ! As always.

SergeBud · Accepted Answer · 2016-01-21 23:26:38Z

0

In the end i got this:

public static void merge(ArrayList<Integer> arr1, ArrayList<Integer> arr2) {
        int indexForArr1 = arr1.size() - 1;
        int oldSize = arr1.size();
        int newSize = arr1.size() + arr2.size();

        /*
            decided not to create new arraylist with new size but just to fill up old one with nulls
         */
        fillWithNulls(arr1, newSize);

        for(int i = (newSize-1); i >= 0; i--) {
            if (i%2 != 0) {
                int indexForArr2 = i%oldSize;
                arr1.set(i,arr2.get(indexForArr2));
                oldSize--; // we reduce the size because we don't need tha last element any more
            } else {
                arr1.set(i, arr1.get(indexForArr1));
                indexForArr1--;
            }
        }
    }

    private static void fillWithNulls(ArrayList<Integer> array, int newSize) {
        int delta = newSize - array.size();
        for(int i = 0; i < delta; i++) {
            array.add(null);
        }
    }

Thanks John again for bright idea!

answered Jan 21, 2016 at 23:26

SergeBud

71 silver badge6 bronze badges

3 Comments

John Sensebe Over a year ago

It'd be a little more efficient to use oldSize - 1 in your for loop and set elements i * 2 and i * 2 + 1 of arr1 in each iteration. You'd get rid of the two %s and the if that way. And use ArrayList.ensureCapacity to efficiently allocate memory for the larger arr1.

SergeBud Over a year ago

i'll try to replace oldsize var later ArrayList.ensureCapacity is obvious, again)

SergeBud Over a year ago

Only now I remembered about put(index, object) method in List collection

Collectives™ on Stack Overflow

Merging two arraylists without creating third one

2 Answers 2

1 Comment

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related