CODEWITHSUNDEEP
javadata-structureslinked-listtraversalsingly-linked-list
hse23
hse23

Reputation: 15

Unable to delete an element from a linked list?

I am just practicing my data structures and trying to make a method to delete duplicates from a singly linked list. This is what I have:

        void removeDup() {
            Node temp = head;
            Node cur = null;
            String s = "";

            while(temp!=null) {
                cur = temp;

                if(!s.contains(temp.data + "")) {
                    s += temp.data + "";
                }
                else {
                    cur.next = temp.next;
                }

                temp = temp.next;
            }
        }

Printing the linked list after executing this method shows no changes. I believe this is because I am not properly linking the previous link to the current link's.next value, but everything looks correct to me. I debug it and it appears to delete the node correctly but the duplicate node still appears when I print out the linked list afterwards. Suggestions?

Upvotes: 0

Views: 231

Answers (3)

wenzi
wenzi

Reputation: 119

The code is copied from https://www.geeksforgeeks.org/remove-duplicates-from-an-unsorted-linked-list/:

Method 1 - brute force,find all pairs of two nodes to see if they have same values, not sure if calling System.gc() is a good idea:

/* Function to remove duplicates from an 
   unsorted linked list */
void remove_duplicates() { 
    Node ptr1 = null, ptr2 = null, dup = null; 
    ptr1 = head; 

    /* Pick elements one by one */
    while (ptr1 != null && ptr1.next != null) { 
        ptr2 = ptr1; 

        /* Compare the picked element with rest 
            of the elements */
        while (ptr2.next != null) { 

            /* If duplicate then delete it */
            if (ptr1.data == ptr2.next.data) { 

                /* sequence of steps is important here */
                dup = ptr2.next; 
                ptr2.next = ptr2.next.next; 
                System.gc(); 
            } else /* This is tricky */ { 
                ptr2 = ptr2.next; 
            } 
        } 
        ptr1 = ptr1.next; 
    } 
}

Method 2 - using hashset to help detect duplicate, I personally like this method better:

 /* Function to remove duplicates from a 
       unsorted linked list */
    static void removeDuplicate(node head)  
    { 
        // Hash to store seen values, changed a little to compile for Java 8
        HashSet<Integer> hs = new HashSet<Integer>(); 

        /* Pick elements one by one */
        node current = head; 
        node prev = null; 
        while (current != null)  
        { 
            int curval = current.val; 

             // If current value is seen before 
            if (hs.contains(curval)) { 
                prev.next = current.next; 
            } else { 
                hs.add(curval); 
                prev = current; 
            } 
            current = current.next; 
        } 

    }

Upvotes: 1

Marc Dzaebel
Marc Dzaebel

Reputation: 435

cur.next = temp.next will not change anything. Use e.g. Java 8:

new LinkedList<>(Arrays.asList(1,2,1,3)).stream().distinct().collect(Collectors.toList());

or

new LinkedHashSet<>(new LinkedList<>(Arrays.asList(1,2,1,3)))

See also https://www.geeksforgeeks.org/remove-duplicates-from-an-unsorted-linked-list

Upvotes: 0

Nicook5
Nicook5

Reputation: 1

First of all I think your choice of holding all the previous things in a single string is probably a bad idea.

For example, if you fed it a list with {x,y, xy}. the third item would be detected as a duplicate. Couple simple alternative methods.
Hold previous values in some collection/ for each element check if anything else is equivalent. sort everything, then check peoples neighbors.

you set cur = temp; at the top of your loop, so doing cur.next = temp.next; afterwards will do nothing. Don't set cur equal to temp at the top of your loop or just change it after.

Upvotes: 0

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