Main.java

class node {
    int data;
    node next;
}
class Slist {
    static node head;
    node tail;
    int count;

    public Slist(){
        head=tail=null;
        count=0;
    }

    public boolean isEmpty(){
        return (head==null);
    }
    public void insertToHead(int val){
        if(isEmpty()){
            head= new node();
            head.data=val;
            head.next=null;
            tail=head;
        }
        else{
            node nn= new node();
            nn.data=val;
            nn.next=head;
            head=nn;
        }
        count++;
    }
    public void insertToTail(int val){
        if(isEmpty())
        {
            insertToHead(val);
            return;
        }
        else{
            node nn=new node();
            nn.data=val;
            nn.next=null;
            tail.next=nn;
            tail=nn;
        }
        count++;
    }
    public void insertToPos(int val, int pos){
        if(pos<1 || pos>count+1)
            System.out.println("wrong position");
        else{
            if(pos==1)
                insertToHead(val);
            else if (pos==count+1)
                insertToTail(val);
            else{
                node nn= new node();
                nn.data=val;
                node t=head;
                for(int i=1; i<pos-1;i++)
                    t=t.next;
                nn.next=t.next;
                t.next=nn;//this line and the one above must be in this exact order.
                count++;
            }


        }
    }
    public int deleteFromHead(){
        int x=-1;//you can put -1000, -999, etc...
        if(!isEmpty())
        {
            x=head.data;// to get back the deleted value if needed.
            if(head==tail)
                head=tail=null;//the whole "array" got deleted.
            else
            {
                node del=head;//this
                head=head.next;
                del=null;//and this line, are to make sure that the data got deleted completely.
                //with out them, the data is just inaccessible but it does exist in the memory.
            }
            count--;
        }
        return x;
    }
    public int deleteFromTail(){
        int x=-1;
        if(!isEmpty())
        {
            x=tail.data;
            if(head==tail)
                head=tail=null;
            else
            {
                node t=head;
                while(t.next!=tail)
                    t=t.next;
                node del=t.next;
                tail=t;
                tail.next=null;//every "array" must ends with null '/0', to let the compiler know that this is the end of the array.
                //if not, then the compiler will continue to print rubbish data after the tail.
                del=null;
            }
            count--;
        }
        return x;
    }
    public int deleteFromPos(int pos){
        int x=-1;
        if(pos<1 || pos>count)//notice that here, pos>count, not count+1, In the other hand InsertToPos was count+1.
            //because we can't delete something that does not exist but we can insert.
            //NOT IMPORTANT LINE//insert(2,8) in array of count 7 means put 2 as an extra node after the already existing tail.
            //NOT IMPORTANT LINE//delete(8) in array of count 7 means delete the 8th node , which is does not exist.
            System.out.println("wrong position");
        else
        {
            if(pos==1)
                return(deleteFromHead());
            else
            if(pos==count)
                return(deleteFromTail());
            else
            {
                node t=head;
                for(int i=1;i<pos-1;i++)
                    t=t.next;
                node del=t.next;
                x=del.data;
                t.next= del.next;
                del=null;
                count--;
            }
        }
        return x;
    }
    public static void print(){
        node t=head;
        while(t!=null){
            System.out.println(t.data);
            t=t.next;
        }
    }
    public int counter (){
        while (node.next)
    }
}
class Main
{
    public static void main(String[] args) {

    }
}