Removing double Objects in Java List

Question

For the exam, I train to write the some algorithms to the final exam. One of them is creating the reverse() method, which removes doubles in the List<> . The problem that the void method only removes one double or two. How can I change the method?

Here, the code of the List.java file. The methods removeFromBack(), removeFromFront(), insertAtBack(), insertAtFront(), print(), isEmpty(), the classes List and ListNode are already defined in Deitel's Java book. Additionally, all imports are done:

public void removeDuplicates() {
    ArrayList<T> toCheck = new ArrayList<T>();
    ListNode<T> current = firstNode;
    while (current != null) {
        toCheck.add(current.data);
        current = current.nextNode;
    }
    current = firstNode;
    HashSet<T> toCheck2 = new LinkedHashSet<T>();

    for (T element: toCheck) {
        toCheck2.add(element);
    }

    for (T element: toCheck2) {
        removeFromBack();
        insertAtBack(element);
    }
}

0xh3xa · Accepted Answer · 2020-05-02 08:55:37Z

1

you can add duplicates list and remove them like this:

    public void removeDuplicates() {
        ArrayList<T> elements = new ArrayList<T>();
        ListNode<T> current = firstNode;
        while (current != null) {
            if(!elements.contains(current.data))
              toCheck.add(current.data);
            else {
               // you have the duplicates, do your logic
            }
            current = current.nextNode;
        }
    }

edited May 2, 2020 at 8:55

answered May 2, 2020 at 2:51

0xh3xa

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1 Comment

Ulvi Bajarani Over a year ago

Yes, but 1) better to add removeFromFront(); after else 2) not use removeFromBack(); 3) and T element : elements, not T element : duplicates. Others are right

Fabian Zimbalev · Accepted Answer · 2020-05-02 03:13:55Z

0

There are faster/more efficient ways depending on your needs but this one should basically always work:

public static <T> ArrayList<T> removeDuplicates(ArrayList<T> toCheck) {

    ArrayList<Integer> toRemove = new ArrayList<Integer>();

    for(int i = 0; i < toCheck.size(); i++) {
        T current = toCheck.get(i);

        for(int j = 0; j < toCheck.size(); j++) {
            if(j == i)continue;

            if(toCheck.get(j) == current) {
                toRemove.add(j);
            }
        }
    }

    Collections.sort(toRemove, Collections.reverseOrder());

    for(int i : toRemove) {
        toCheck.remove(i);
    }

    return toCheck;

}

answered May 2, 2020 at 3:13

Fabian Zimbalev

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1 Comment

Ulvi Bajarani Over a year ago

I know that it is possible to use either native methods or arguments, but no arguments are needed.

PatrickChen · Accepted Answer · 2020-05-02 03:20:13Z

0

For duplicates in List, you could always use Set to help to remove the duplicates:


public List<Integer> removeDuplicates(List<Integer> list) {
    Set<Integer> set = new HashSet<>(list); // remove all duplicates in set
    List<Integer> result = new ArrayList(set.size());
    for (Integer i : list) {
         if (set.contains(i)) {
             result.add(i);
             set.remove(i); // delete, so duplicate item will not be added to result twice
         }
    }

    return result;
}

answered May 2, 2020 at 3:20

PatrickChen

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2 Comments

Ulvi Bajarani Over a year ago

I knew, but the case that it should work with non-primitive types, which HashSet couldn't describe.

PatrickChen Over a year ago

Set could work on objects, as long as you override the equals and hashcode method in this object.

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