数据结构 Java循环双向链表

■　构造函数
每个构造函数都通过 this 来初始化链接域 prev 和 next．图⑴

prev = this; next = this;

■　判断表空的条件

public boolean isEmpty() {
		return (header.prev == header || header.next == header);
}

图⑴



■　addBefore 方法是 MyLinkedList 类的重要方法, 后面的很多方法都要依靠它来实现！图⑵

	private static <T> DNode<T> addBefore(DNode<T> curr, T item) {
		DNode<T> newNode, prevNode;

		newNode = new DNode<T>(item);
		
		prevNode = curr.prev;
		
		newNode.prev = prevNode;　①
		newNode.next = curr;　②
		
		prevNode.next = newNode;　③
		curr.prev = newNode;　④
     return newNode;
	}

图⑵


■　addFirst方法就是靠addBefore来实现的！
因为
header.next = nd'  -->图⑵
所以
curr = nd'  --> addBefore()方法
因此
prevNode = curr.prev = h  --> addBefore()方法
继续重复这四句话
newNode.prev = prevNode;　①
newNode.next = curr;　②
prevNode.next = newNode;　③
curr.prev = newNode;　④

public void addFirst(T item) {
		addBefore(header.next, item);
		listSize++;
}

图⑶


■　remove方法也很重要！

private void remove(DNode<T> curr) {
		if(curr.next == curr) return;
		
		DNode<T> prevNode = curr.prev, nextNode = curr.next;
		
		prevNode.next = nextNode;
		nextNode.prev= prevNode;
}

package LinkedList;

import java.util.Iterator;
import java.util.ListIterator;
import java.util.NoSuchElementException;

public class MyLinkedList<T> {
	//*************************************************************
	private DNode<T> header;
	private int listSize;
	//*************************************************************
	public MyLinkedList() {
		header = new DNode<T>();
		listSize = 0;
	}
	//*************************************************************
	private static class DNode<T> {
		T nodeValue;
		DNode<T> prev;
		DNode<T> next;

		public DNode() { // for header
			nodeValue = null;
			prev = this; // left
			next = this; // right
		}

		public DNode(T item) {
			nodeValue = item;
			prev = this;
			next = this;
		}
	}
	//**************************************************************
	public boolean isEmpty() {
		return (header.prev == header || header.next == header);
	}
	
	public int size() {
		return listSize;
	}
	//**************************************************************
	private DNode<T> addBefore(DNode<T> curr, T item) {
		DNode<T> newNode, prevNode;

		newNode = new DNode<T>(item);
		
		prevNode = curr.prev;
		
		newNode.prev = prevNode;
		newNode.next = curr;
		
		prevNode.next = newNode;
		curr.prev = newNode;

		return newNode;
	}

	public boolean add(T item) {
		addBefore(header, item);
		listSize++;
		return true;
	}
	
	public void addFirst(T item) {
		addBefore(header.next, item);
		listSize++;
	}
	
	public void addLast(T item) {
		addBefore(header, item);
		listSize++;
	}
	//**************************************************************
	private void remove(DNode<T> curr) {
		if(curr.next == curr) return;
		
		DNode<T> prevNode = curr.prev, nextNode = curr.next;
		
		prevNode.next = nextNode;
		nextNode.prev= prevNode;
	}
	
	public boolean remove(Object o) {
		for(DNode<T> p = header.next; p != header; p = p.next) {
			if(o.equals(p.nodeValue)) {
				remove(p);
				listSize--;
				return true;
			}
		}
		return false;
	}
	//**************************************************************
	public void printList() {
		for(DNode<T> p = header.next; p != header; p = p.next)
			System.out.println(p.nodeValue);
	}
	//**************************************************************
	private class MyIterator implements Iterator<T> {
		
		public DNode<T> nextNode = header.next;
		public DNode<T> lastReturned = header;
		
		public boolean hasNext() {
			return nextNode != header;
		}
		
		public T next() {
			if(nextNode == header)
				throw new NoSuchElementException("");
			
			lastReturned = nextNode;
			nextNode = nextNode.next;
			return lastReturned.nodeValue;
		}

		public void remove() {
			if(lastReturned == header)
				throw new IllegalStateException("");
			
			MyLinkedList.this.remove(lastReturned);
			lastReturned = header;
			listSize--;
		}
	}
	//**************************************************************
	private class MyListIterator extends MyIterator implements ListIterator<T> {
		
		private int nextIndex;
		
		MyListIterator(int index) {
			if(index < 0 || index > listSize)
				throw new IndexOutOfBoundsException("");
			
			//如果index小于listSize/2，就从表头开始查找定位，否则就从表尾开始查找定位
			if(index < (listSize >> 1)) {
				nextNode = header.next;
				for(nextIndex = 0; nextIndex < index; nextIndex++)
					nextNode = nextNode.next;
			}else {
				nextNode = header;
				for(nextIndex = listSize; nextIndex > index; nextIndex--)
					nextNode = nextNode.prev;
			}
		}

		public boolean hasPrevious() {
			return nextIndex != 0;
			//return nextNode.prev != header;
		}
		
		public T previous() {
			if (nextIndex == 0)
				throw new NoSuchElementException("no");
			
			lastReturned = nextNode = nextNode.prev;
			nextIndex--;
			return lastReturned.nodeValue;
		}
		
		public void remove() {
			if(lastReturned == header)
				throw new IllegalStateException("");
			
			MyLinkedList.this.remove(lastReturned);
			nextIndex--;
			listSize--;
			
			if(lastReturned == nextNode)
				nextNode = nextNode.next;
			lastReturned = header;
		}
		
		public void add(T item) {
			MyLinkedList.this.addBefore(nextNode, item);
			nextIndex++;
			listSize++;
			lastReturned = header;
		}
		
		public void set(T item) {
			 if (lastReturned == header)
				 throw new IllegalStateException();
			 
			 lastReturned.nodeValue = item;
		}
		
		public int nextIndex() {
			return nextIndex;
		}

		public int previousIndex() {
			return nextIndex - 1;
		}
	}
	
	//**************************************************************
	public Iterator<T> iterator() {
		return new MyIterator();
	}
	//**************************************************************
	public ListIterator<T> listIterator(int index) {
		return new MyListIterator(index);
	}
	//**************************************************************
	public static void main(String[] args) {
		MyLinkedList<String> t = new MyLinkedList<String>();
		t.add("A");
		t.add("B");
		t.add("C");
		t.add("D");
		//t.remove("B");
		//t.addFirst("AA");
		//t.addLast("BB");
		//t.printList();
		/*
		Iterator<String> it = t.iterator();
		while(it.hasNext()) System.out.println(it.next()); // A B C D
		*/
		
		ListIterator<String> it = t.listIterator(t.size());
		
		while(it.hasPrevious()) {
			System.out.println(it.previous()); // D C B A
		}
	}

}// MyLinkedList end~