编写双向循环链表，实现：追加、插入、删除、遍历

##直接源代码

public class LinkedList<E>{
	private Node head;
	private int size;
	private class Node{
		E data;
		Node prev;
		Node next;
		public Node(E e) {
			data = e;
		}
	}
	//向集合中追加元素
	public boolean add(E e) {
		if(head==null) {
			head = new Node(e);
			//建立双向循环关系
			head.next = head;
			head.prev = head;
		}else {//第二个以后的节点
			Node last = head.prev;
			Node node = new Node(e);
			last.next = node;
			node.next = head;
			head.prev = node;
			node.prev = last;
		}
		size++;
		return true;
	}
	
	public int size() {
		return size;
	}
	
	public E get(int index) {
		if(index<0 || index > size-1) {
			throw new IndexOutOfBoundsException("越界!"+index);
		}
		return find(index).data;
	}
	
	public E remove(int index) {
		if(index<0 || index > size-1) {
			throw new IndexOutOfBoundsException("越界!"+index);
		}
		if(size==1 && index==0) {
			E e = head.data;
			head=null;
			size=0;
			return e;
		}
		Node node = find(index);
		Node prev = node.prev;
		Node next = node.next;
		prev.next = next;
		next.prev = prev;
		node.next = null;
		node.prev = null;
		
		if(index==0) {
			head=next;
		}
		
		size--;
		return node.data;
	}
	
	public boolean add(int index, E e) {
		if(index<0 || index>size) {
			throw new IndexOutOfBoundsException("越界!"+index);
		}
		if(index==size) {
			return add(e);
		}
		Node node = find(index);
		Node prev = node.prev;
		Node next = node;
		Node n = new Node(e);
		prev.next = n;
		n.next = next;
		next.prev = n;
		n.prev = prev;
		if(index==0) {
			head=n;
		}
		size++;
		return true;
	}

	private Node find(int index) {
		Node node;
		if(index < size/2) {
			node = head;
			for (int i = 0; i < index; i++) {
				node = node.next;
			}
		}else {
			node=head.prev;
			for(int i=size-1;i>index;i--){
				node=node.prev;
			}
		}
		return node;
	}

	public String toString() {
		// [] 
		// [8, 9, 5, 6, 7] 
		if(head==null) {
			return "[]";
		}
		StringBuilder buf = new StringBuilder("[");
		buf.append(head.data);
		Node node = head.next;
		while(node != head) {
			buf.append(", ").append(node.data);
			node = node.next;
		}
		return buf.append("]").toString();
	}
}

##测试代码

public class LinkedListTest {

	public static void main(String[] args) {
		LinkedList<String> list = 
				new LinkedList<String>();
		list.add("Tom");
		list.add("Jerry");
		list.add("Andy");
		list.add("Mac");
		list.add("Wang");
		System.out.println(list); 
		list.add(1,"光头强");
		System.out.println(list); 
		list.add(4, "熊大");
		list.add(0, "熊二");
		System.out.println(list); 
		list.remove(0);
		System.out.println(list);
	}
}