How can I merge two lexicographically ordered String ArrayLists into a new third ArrayList?

Question

How can I merge two lexicographically ordered String ArrayLists into a new third ArrayList without modifying the two original lists? Duplicates are allowed, for instance. If list1 is: 1, 3, 5 and list2 is: 2, 4, 7, 8, 8. Then the new merged list3 should be: 1, 2, 3, 4, 5, 7, 8, 8. I need to create 2 while loops, the first one keeps going until through a whole list, and the second one takes the remains from the other list and puts all the leftover elements at the end of the new list3. I basically need to compare both list indexes to see which element advances. Everything should be correct but I need help making the while loops.

Sorry for the long description, but I will up-vote anyone that helps! Thank you!

public class Merge{
    public static boolean isStringArrayListSorted(ArrayList<String> data) {
        for (int i = 1; i < data.size(); i++) {
            if (data.get(i - 1).compareTo(data.get(i)) > 0) {
                return false;
            }
        }
        return true;
    }

    public Merge (ArrayList<String> list1, ArrayList<String> list2){

        ArrayList<String> list3 = new ArrayList<String>();

    list1.add(0, "a");
    list1.add(1, "c");
    list1.add(2, "e");
    list1.add(3, "g");

    list2.add(0, "b");
    list2.add(1, "d");
    list2.add(2, "f");
    list2.add(3, "h");
    list2.add(4, "i");
    list2.add(5, "j");

    while(list1!= null){
        if(list1.get(0)> list2.get(0))
        {
            list3.add(index, element);
        }
    }
}

fyi, you don't need the indices in your list add() calls, the single argument add() methods appends to the end. — Roman, Apr 14 '16 at 03:55

score 2 · Answer 1 · answered Apr 14 '16 at 03:53

2

Why not to use Collections.sort()?

ArrayList<String> list3 = new ArrayList<String>(list1);
list3.addAll(list2);
Collections.sort(list3);

answered Apr 14 '16 at 03:53

Nurlan

720
7
12

Well I would like to create an algorithm to do this with while loops by comparing indexes to determine which element advances into the new list. – MercedezB Apr 14 '16 at 03:57
Nowadays CPUs are very fast, so an O(n log n) sort may not worth to concern in compare to an O(n) merge [with the assumption that there's no cost of obtaining the initial sorted list]. So... ^_^ – Ken Cheung Apr 15 '16 at 02:11

score 2 · Accepted Answer · answered Apr 14 '16 at 03:58

Assume you really want to do a merge rather than combine the two and sort again, its a simple merge code like this:

public static ArrayList<String> merge(ArrayList<String> L1, ArrayList<String> L2) {
    int i1 = 0, i2 = 0;
    ArrayList<String> result = new ArrayList<String>();

    while (i1 < L1.size() && i2 < L2.size()) {
        if (L1.get(i1).compareTo(L2.get(i2)) < 0) {
            result.add(L1.get(i1));
            i1++;
        } else {
            result.add(L2.get(i2));
            i2++;
        }
    }

    while (i1 < L1.size()) {
        result.add(L1.get(i1));
        i1++;
    }

    while (i2 < L2.size()) {
        result.add(L2.get(i2));
        i2++;
    }

    return result;
}

ArrayList which implements AbstractList allows random access to the elements in the middle while LinkedList implements AbstractSequentialList (which extends AbstractList) only impose an additional "sequential access" only. In my code I didn't manipulate entries in the middle, thus the ArrayList will still preserve the order. — Ken Cheung, Apr 14 '16 at 04:13

score 1 · Answer 3 · answered Apr 14 '16 at 03:56

1

I would suggest using Collections.sort () here. Just create a new list, add all elements and sort it. The chide would be shorter, more readable and you will be using the high preformance built in sorting algorithm.

answered Apr 14 '16 at 03:56

Danail Alexiev

7,624
3
20
28

How can I merge two lexicographically ordered String ArrayLists into a new third ArrayList?

3 Answers3