public class LinkedList<T>{

  public int size;
  Node<T> head, tail;

  private class Node<X>
  {
    Node(X _data){ data=_data; }
    X data;
    Node<X> next;
    Node<X> pre;
  }

  // Constructor to create an empty list
  public LinkedList( ){
    size=0;
  }


  // Add x at position i
  public void insert(int i, T x)
  {
    if(i<0 || i>=size){
      throw new IndexOutOfBoundsException("shit this is out of bound: "+i);
    }

    Node<T> n=this.head;
    for(int j=0;j<i;j++) n=n.next;

    //
    Node<T> m=new Node<T>(x);
    m.pre=n;
    m.next=n.next;
    n.next=m;
    if(m.next!=null) m.next.pre=m;

    //
    if(n==tail) tail=m;

    size++;
  }

  //Add x at the end
  public void add(T x)
  {
    Node<T> n=new Node<T>(x);

    if(tail!=null)
    {
      tail.next=n;
      n.pre=tail;
      tail=n;
    }
    else{
      head=tail=n;
    }

    size++;
  }

  //get the i-th element
  public T get(int i)
  {
    if(i<0 || i>=size) return null;
    Node<T> n=this.head;
    for(int j=0;j<i;j++) n=n.next;

    return n.data;
  }

  public void set(int i, T x){

    if(i<0 || i>=size){
      throw new IndexOutOfBoundsException("shit this is out of bound: "+i);
    }

    Node<T> n=this.head;
    for(int j=0;j<i;j++) n=n.next;

    n.data=x;
  }

  public void remove(int i)
  {
    if(i<0 || i>=size){
      throw new IndexOutOfBoundsException("shit this is out of bound: "+i);
    }

    Node<T> n=this.head;
    for(int j=0;j<i;j++) n=n.next;

    //delete n
    if(n.pre!=null) n.pre.next=n.next;
    if(n.next!=null) n.next.pre=n.pre;

    if(n==tail) tail=n.pre;
    if(n==head) head=n.next;

    size--;
  }

}