//双端队列
//大小
transient int size = 0;
//第一个节点元素
transient Node<E> first;
//最后一个节点元素
transient Node<E> last;

public LinkedList() {
}

private static class Node<E> {
E item;
Node<E> next;
Node<E> prev;

Node(Node<E> prev, E element, Node<E> next) { this.item = element; this.next = next; this.prev = prev; }}

//在首位追加
private void linkFirst(E e) {
final Node<E> f = first;
final Node<E> newNode = new Node<>(null, e, f);
first = newNode;
if (f == null)
last = newNode;
else
f.prev = newNode;
size++;
modCount++;
}

/** * Links e as last element. */ //在末位追加void linkLast(E e) { final Node<E> l = last; final Node<E> newNode = new Node<>(l, e, null); last = newNode; if (l == null) first = newNode; else l.next = newNode; size++; modCount++;}/** * Inserts element e before non-null Node succ. */ //在指定节点前添加元素void linkBefore(E e, Node<E> succ) { // assert succ != null; final Node<E> pred = succ.prev; final Node<E> newNode = new Node<>(pred, e, succ); succ.prev = newNode; if (pred == null) first = newNode; else pred.next = newNode; size++; modCount++;}/** * Unlinks non-null first node f. */private E unlinkFirst(Node<E> f) { // assert f == first && f != null; final E element = f.item; final Node<E> next = f.next; f.item = null; f.next = null; // help GC first = next; if (next == null) last = null; else next.prev = null; size--; modCount++; return element;}/** * Unlinks non-null last node l. */private E unlinkLast(Node<E> l) { // assert l == last && l != null; final E element = l.item; final Node<E> prev = l.prev; l.item = null; l.prev = null; // help GC last = prev; if (prev == null) first = null; else prev.next = null; size--; modCount++; return element;}

//添加元素
public boolean add(E e) {
//在链表最后一个节点追加一个节点
linkLast(e);
return true;
}