数据结构（Java语言）——LinkedList简单实现

下面是一个能够使用的LinkedList泛型类的实现。这里的链表类名为MyLinkedList，避免与类库中反复。

MyLinkedList将作为双链表实现，并且保留到该表两端的引用。这样仅仅要操作发生在已知的位置，就能够保持每一个操作花费常数时间的代价。这个已知的位置能够是端点。也能够是由迭代器指定的一个位置。

设计方面，主要分为三个部分实现：

1. MyLinkedList类本身，包括到两端的链、表的大小以及一些方法。
2. Node类，它可能是一个私有的嵌套类。一个节点包括数据以及到前一个节点的链和到下一个节点的链，另一些适当的构造方法。
3. LinkedListIterator类，该类抽象了位置的概念，是一个私有类，并实现接口Iterator。
   
   包括方法next()，hasNext()，remove()的实现。

import java.util.ConcurrentModificationException;
import java.util.Iterator;
import java.util.NoSuchElementException;

public class MyLinkedList<AnyType> implements Iterable<AnyType> {
	private int theSize;
	private int modCount = 0;
	private Node<AnyType> beginMarker;
	private Node<AnyType> endMarker;

	private static class Node<AnyType> {
		public AnyType data;
		public Node<AnyType> prev;
		public Node<AnyType> next;

		public Node(AnyType d, Node<AnyType> p, Node<AnyType> n) {
			data = d;
			prev = p;
			next = n;
		}
	}

	public MyLinkedList() {
		clear();
	}

	public void clear() {
		beginMarker = new Node<AnyType>(null, null, null);
		endMarker = new Node<AnyType>(null, beginMarker, null);
		beginMarker.next = endMarker;
		theSize = 0;
		modCount++;
	}

	public int size() {
		return theSize;
	}

	public boolean isEmpty() {
		return size() == 0;
	}

	public void add(AnyType x) {
		add(size(), x);
	}

	public void add(int idx, AnyType x) {
		addBefore(getNode(idx), x);
	}

	public AnyType get(int idx) {
		return getNode(idx).data;
	}

	public AnyType set(int idx, AnyType newVal) {
		Node<AnyType> p = getNode(idx);
		AnyType oldVal = p.data;
		p.data = newVal;
		return oldVal;
	}

	public AnyType reomve(int idx) {
		return remove(getNode(idx));
	}

	private void addBefore(Node<AnyType> p, AnyType x) {
		Node<AnyType> newNode = new Node<AnyType>(x, p.prev, p);
		newNode.prev.next = newNode;
		p.prev = newNode;
		theSize++;
		modCount++;
	}

	private AnyType remove(Node<AnyType> p) {
		p.next.prev = p.prev;
		p.prev.next = p.next;
		theSize--;
		modCount++;
		return p.data;
	}

	private Node<AnyType> getNode(int idx) {
		Node<AnyType> p;
		if (idx < 0 || idx > size()) {
			throw new IndexOutOfBoundsException();
		}
		if (idx < size() / 2) {
			p = beginMarker.next;
			for (int i = 0; i < idx; i++) {
				p = p.next;
			}
		} else {
			p = endMarker;
			for (int i = size(); i < idx; i--) {
				p = p.prev;
			}
		}
		return p;
	}

	public Iterator<AnyType> iterator() {
		return new LinkedListIterator();
	}

	private class LinkedListIterator implements Iterator<AnyType> {
		private Node<AnyType> current = beginMarker.next;
		private int expectedModCount = modCount;
		private boolean okToRemove = false;

		@Override
		public boolean hasNext() {
			return current != endMarker;
		}

		@Override
		public AnyType next() {
			if (modCount != expectedModCount) {
				throw new ConcurrentModificationException();
			}
			if (!hasNext()) {
				throw new NoSuchElementException();
			}
			AnyType nextItem = current.data;
			current = current.next;
			okToRemove = true;
			return nextItem;
		}

		public void remove() {
			if (modCount != expectedModCount) {
				throw new ConcurrentModificationException();
			}
			if (!okToRemove) {
				throw new IllegalStateException();
			}
			MyLinkedList.this.remove(current.prev);
			okToRemove = false;
			expectedModCount++;
		}
	}
}