链表

public class MyLinkedList<E> extends MyAbstractList<E> {
  private Node<E> head, tail;

  /** Create a default list */
  public MyLinkedList() {
  }

  /** Create a list from an array of objects */
  public MyLinkedList(E[] objects) {
    super(objects);
  }

  /** Return the head element in the list */
  public E getFirst() {
    if (size == 0) {
      return null;
    }
    else {
      return head.element;
    }
  }

  /** Return the last element in the list */
  public E getLast() {
    if (size == 0) {
      return null;
    }
    else {
      return tail.element;
    }
  }

  /** Add an element to the beginning of the list */
  public void addFirst(E e) {
    Node<E> newNode = new Node<E>(e); // Create a new node
    newNode.next = head; // link the new node with the head
    head = newNode; // head points to the new node
    size++; // Increase list size

    if (tail == null) // the new node is the only node in list
      tail = head;
  }

  /** Add an element to the end of the list */
  public void addLast(E e) {
    Node<E> newNode = new Node<E>(e); // Create a new for element e

    if (tail == null) {
      head = tail = newNode; // The new node is the only node in list
    }
    else {
      tail.next = newNode; // Link the new with the last node
      tail = tail.next; // tail now points to the last node
    }

    size++; // Increase size
  }


  /** Add a new element at the specified index in this list
   * The index of the head element is 0 */
  public void add(int index, E e) {
    if (index == 0) {
      addFirst(e);
    }
    else if (index >= size) {
      addLast(e);
    }
    else {
      Node<E> current = head;
      for (int i = 1; i < index; i++) {
        current = current.next;
      }
      Node<E> temp = current.next;
      current.next = new Node<E>(e);
      (current.next).next = temp;
      size++;
    }
  }

  /** Remove the head node and
   *  return the object that is contained in the removed node. */
  public E removeFirst() {
    if (size == 0) {
      return null;
    }
    else {
      Node<E> temp = head;
      head = head.next;
      size--;
      if (head == null) {
        tail = null;
      }
      return temp.element;
    }
  }

  /** Remove the last node and
   * return the object that is contained in the removed node. */
  public E removeLast() {
    if (size == 0) {
      return null;
    }
    else if (size == 1) {
      Node<E> temp = head;
      head = tail = null;
      size = 0;
      return temp.element;
    }
    else {
      Node<E> current = head;

      for (int i = 0; i < size - 2; i++) {
        current = current.next;
      }

      Node<E> temp = tail;
      tail = current;
      tail.next = null;
      size--;
      return temp.element;
    }
  }

  /** Remove the element at the specified position in this list.
   *  Return the element that was removed from the list. */
  public E remove(int index) {
    if (index < 0 || index >= size) {
      return null;
    }
    else if (index == 0) {
      return removeFirst();
    }
    else if (index == size - 1) {
      return removeLast();
    }
    else {
      Node<E> previous = head;

      for (int i = 1; i < index; i++) {
        previous = previous.next;
      }

      Node<E> current = previous.next;
      previous.next = current.next;
      size--;
      return current.element;
    }
  }

  /** Override toString() to return elements in the list */
  public String toString() {
    StringBuilder result = new StringBuilder("[");

    Node<E> current = head;
    for (int i = 0; i < size; i++) {
      result.append(current.element);
      current = current.next;
      if (current != null) {
        result.append(", "); // Separate two elements with a comma
      }
      else {
        result.append("]"); // Insert the closing ] in the string
      }
    }

    return result.toString();
  }

  /** Clear the list */
  public void clear() {
    head = tail = null;
  }

  /** Return true if this list contains the element o */
  public boolean contains(E e) {
    Node<E> previous = head;

    for (int i = 1; i < size; i++) {
        if(previous.element == e)
            return true;
        previous = previous.next;
    } 
    return false;
  }

  /** Return the element from this list at the specified index */
  public E get(int index) {
      if(size == 0) return null;
      else if(index < 0 || index >= size )return null;
      else
      {
          Node<E> previous = head;
          for(int i = 1; i <= index; i++)
              previous = previous.next;
          return previous.element;
      }
  }

  /** Return the index of the head matching element in this list.
   *  Return -1 if no match. */
  public int indexOf(E e) {
     
    Node<E> previous = head;
    for(int i = 0; i < size; i++){
        if(previous.element == e)
            return i;
        previous = previous.next;
      }
    return -1;
  }

  /** Return the index of the last matching element in this list
   *  Return -1 if no match. */
  public int lastIndexOf(E e) {
      int index = -1;
      Node<E> previous = head;
      for(int i = 0; i < size; i++){
          if(previous.element == e)
             index  = i;
          previous = previous.next;
      }
      return index;
  }

  /** Replace the element at the specified position in this list
   *  with the specified element. */
  public E set(int index, E e) {
      if(index < 0 || index > size) return null;
      if(index == 0) {
          E old = head.element;
          head.element = e;
          return old;
      }
      else 
          {
              Node<E> newNode = new Node<E>(e);
              Node<E> previous = head;
              for(int i = 1; i < index; i++)
                  previous = previous.next;
              Node<E> temp = previous.next;
              E old = temp.element;
              newNode.next = temp.next;
              previous.next = newNode;
              return old;
          }
      
  }

  private static class Node<E> {
    E element;
    Node<E> next;

    public Node(E element) {
      this.element = element;
    }
  }
}