Removing a specific item from LinkedList in java without using any API

Question

I am trying to remove an item from the LinkedList in java. This List is implemented by me and I am not using any java API. The major trouble I am facing is with RECURSION as I am always lost in recursion coding.

class List{

    int N;
    List next;
    List current;
    List(int N){
        this.N =N;
        this.next = null;
    }

    @Override
    public String toString() {
        String o = "";
        List curr = this;
        while(curr != null){
            o += curr.N+"-->";
            curr = curr.next;
        }

        return o+"TAIL";
    }
}

Method implemented:

private static List Remove(List L,int N){
    if(L == null || L.next == null)
        return L;

    List current = L;
    List previous = null;

    while(current != null){
        if(current.N == N){
            current = current.next;
            if(previous == null)previous = current;
            else{
                previous.next = current;
            }
            break;
        }else{
            previous = current;
            current = current.next;             
        }   
    }
    return previous;
}

Input -

List list1 = new List(1);
        list1.next = new List(2);
        list1.next.next = new List(3);
        list1.next.next.next  = new List(4);
        list1.next.next.next.next  = new List(5);
        list1.next.next.next.next.next  = new List(6);
        list1.next.next.next.next.next.next  = new List(7);
System.out.println("Before Removal "+list1.toString());
        System.out.println("After Removal "+Remove(list1,3));

Output I am getting is -

• Before Removal 1-->2-->3-->4-->5-->6-->7-->TAIL
• After Removal 2-->4-->5-->6-->7-->TAIL

Here I am losing the value 1 as I am setting the current = current.next or reference is being set to next value. So definitely I am having some problem with the presentation of data stored in different references.

Answer 1

It's simply because you are not returning the head - but instead the previous pointer to the node you just 'removed':

static List Remove(final List L, final int N) {
    // Base case for null head pointer  
    final List head = L;
    if (head == null)
        return head;

    // Base case for removing the head
    if (head.N == N)
       return head.next;

    List current = head.next;
    List previous = head;

    while (current != null) {
        if (current.N == N) {
            current = current.next;
            if (previous == null) {
                previous = current;
            }
            else {
                previous.next = current;
            }

            break;
        } else {
            previous = current;
            current = current.next;
        }
    }

    return head;
}

Also - to clarify - this is not a recursive solution.

Answer 2

The mistake is here:

return previous;

You should return the original head of the list if it was not removed. To show it graphically:

N == 3
List Before Removal: 1-->2-->3-->4-->5-->6-->7-->TAIL
At start of iteration 1:
L                    ^
previous      (null)
current              ^
No match -> iteration 2:
L                    ^
previous             ^
current                  ^
No match -> iteration 3:
L                    ^
previous                 ^
current                      ^
Match -> remove current:
List After Removal:  1-->2-->4-->5-->6-->7-->TAIL
L                    ^
previous                 ^
current                      ^

At this point by returning previous, you lose the former head element L.

For the case when the head element is to be removed, you should add a separate check before the loop.

Btw your Remove method is not recursive - it is never calling itself.